During the first Republican debate of the 2024 presidential primary campaign last month, Donald Trump’s rivals were asked to raise their hands if they would support his candidacy, even if he were “convicted in a court of law.” Mr. Trump’s effort to overturn the 2020 election wasn’t just a potential criminal offense. It also violated the cardinal rule of democracy: Politicians must accept the results of elections, win or lose.But that seemed to matter little on the debate stage. Vivek Ramaswamy’s hand shot up first, and all the other leading candidates followed suit — some eagerly, some more hesitantly and one after casting furtive glances to his right and his left.Behavior like this might seem relatively harmless — a small act of political cowardice aimed at avoiding the wrath of the base. But such banal acquiescence is very dangerous. Individual autocrats, even popular demagogues, are never enough to wreck a democracy. Democracy’s assassins always have accomplices among mainstream politicians in the halls of power. The greatest threat to our democracy comes not from demagogues like Mr. Trump or even from extremist followers like those who stormed the Capitol on Jan. 6, but rather from the ordinary politicians, many of them inside the Capitol that day, who protect and enable him.The problem facing Republican leaders today — the emergence of a popular authoritarian threat in their own ideological camp — is hardly new. It has confronted political leaders across the world for generations. In Europe in the 1920s and 1930s, mainstream center-left and center-right parties had to navigate a political world in which antidemocratic extremists on the communist left and the fascist right enjoyed mass appeal. And in much of South America in the polarized 1960s and 1970s, mainstream parties found that many of their members sympathized with either leftist guerrillas seeking armed revolution or rightist paramilitary groups pushing for military rule.The Spanish political scientist Juan Linz wrote that when mainstream politicians face this sort of predicament, they can proceed in one of two ways.On the one hand, politicians may act as loyal democrats, prioritizing democracy over their short-term ambitions. Loyal democrats publicly condemn authoritarian behavior and work to hold its perpetrators accountable, even when they are ideological allies. Loyal democrats expel antidemocratic extremists from their ranks, refuse to endorse their candidacies, eschew all collaboration with them, and when necessary, join forces with ideological rivals to isolate and defeat them. And they do this even when extremists are popular among the party base. The result, history tells us, is a political firewall that can help a democracy survive periods of intense polarization and crisis.On the other hand, too often, politicians become what Mr. Linz called semi-loyal democrats. At first glance, semi-loyalists look like loyal democrats. They are respectable political insiders and part of the establishment. They dress in suits rather than military camouflage, profess a commitment to democracy and ostensibly play by its rules. We see them in Congress and in governor’s mansions — and on the debate stage. So when democracies die, semi-loyalists’ fingerprints may not be found on the murder weapon.But when we look closely at the histories of democratic breakdowns, from Europe in the interwar period to Argentina, Brazil and Chile in the 1960s and 1970s to Venezuela in the early 2000s, we see a clear pattern: Semi-loyal politicians play a pivotal role in enabling authoritarians.Rather than severing ties to antidemocratic extremists, semi-loyalists tolerate and accommodate them. Rather than condemn and seek accountability for antidemocratic acts committed by ideological allies, semi-loyalists turn a blind eye, denying, downplaying and even justifying those acts — often via what is today called whataboutism. Or they simply remain silent. And when they are faced with a choice between joining forces with partisan rivals to defend democracy or preserving their relationship with antidemocratic allies, semi-loyalists opt for the latter.It is semi-loyalists’ very respectability that makes them so dangerous. As members of the establishment, semi-loyalists can use their positions of authority to normalize antidemocratic extremists, protect them against efforts to hold them legally accountable and empower them by opening doors to the mainstream media, campaign donors and other resources. It is this subtle enabling of extremist forces that can fatally weaken democracies.Consider the example of France. On Feb. 6, 1934, in the center of Paris, thousands of disaffected and angry men — veterans and members of right-wing militia groups — gathered near the national Parliament as its members were inside preparing to vote for a new government. They threw chairs, metal grates and rocks and used poles with razor blades on one end to try breach the doors of Parliament. Members of Parliament, frightened for their lives, had to sneak out of the building. Seventeen people were killed, and thousands were injured. Although the rioters failed to seize the Parliament building, they achieved one of their objectives: The centrist prime minister resigned the next day and was replaced by a right-leaning prime minister.Although French democracy survived the Feb. 6 attack on Parliament, the response of some prominent politicians weakened its defenses. Many centrist and center-left politicians responded as loyal democrats, publicly and unequivocally condemning the violence. But many conservative politicians did not. Key members of France’s main conservative party, the Republican Federation, many of whom were inside the Parliament building that day, sympathized publicly with the rioters. Some praised the insurrectionists as heroes and patriots. Others dismissed the importance of the attack, denying that there had been an organized plot to overthrow the government.When a parliamentary commission was established to investigate the events of Feb. 6, Republican Federation leaders sabotaged the investigation at each step, blocking even modest efforts to hold the rioters to account. Protected from prosecution, many of the insurrection’s organizers were able to continue their political careers. Some of the rioters went on to form the Victims of Feb. 6, a fraternity-like organization that later served as a recruitment channel for the Nazi-sympathizing Vichy government established in the wake of the 1940 German invasion.The failure to hold the Feb. 6 insurrectionists to account also helped legitimize their ideas. Mainstream French conservatives began to embrace the view — once confined to extremist circles — that their democracy was hopelessly corrupt, dysfunctional and infiltrated by Communists and Jews. Historically, French conservatives had been nationalist and staunchly anti-German. But by 1936, many of them so despised the Socialist prime minister, Léon Blum, that they embraced the slogan “Better Hitler than Blum.” Four years later, they acquiesced to Nazi rule.The semi-loyalty of leading conservative politicians fatally weakened the immune system of French democracy. The Nazis, of course, finished it off.A half-century later, Spanish politicians responded very differently to a violent assault on Parliament. After four decades of dictatorship, Spain’s democracy was finally restored in the late 1970s, but its early years were marked by economic crisis and separatist terrorism. And on Feb. 23, 1981, as the Parliament was electing a new prime minister, 200 civil guardsmen entered the building and seized control at gunpoint, holding the 350 members of Parliament hostage. The coup leaders hoped to install a conservative general — a kind of Spanish Charles de Gaulle — as prime minister.The coup attempt failed, thanks to the quick and decisive intervention of the king, Juan Carlos I. Nearly as important, though, was the reaction of Spanish politicians. Leaders across the ideological spectrum — from communists to conservatives who had long embraced the Franco dictatorship — forcefully denounced the coup. Four days later, more than a million people marched in the streets of Madrid to defend democracy. At the head of the rally, Communist, Socialist, centrist and conservative franquista politicians marched side by side, setting aside their partisan rivalries to jointly defend democracy. The coup leaders were arrested, tried and sentenced to long prison terms. Coups became virtually unthinkable in Spain, and democracy took root.That is how democracy is defended. Loyal democrats join forces to condemn attacks on democracy, isolate those responsible for such attacks and hold them accountable.Unfortunately, today’s Republican Party more closely resembles the French right of the 1930s than the Spanish right of the early 1980s. Since the 2020 election, Republican leaders have enabled authoritarianism at four decisive moments. First, rather than adhering to the cardinal rule of accepting election results after Joe Biden won in November, many Republican leaders either questioned the results or remained silent, refusing to publicly recognize Mr. Biden’s victory. Vice President Mike Pence did not congratulate his successor, Kamala Harris, until the middle of January 2021. The Republican Accountability Project, a Republican pro-democracy watchdog group, evaluated the public statements of 261 Republican members of the 117th Congress after the election. They found that 221 of them had publicly expressed doubt about its legitimacy or did not publicly recognize that Biden won. That’s 85 percent. And in the aftermath of the Jan. 6 riot, nearly two-thirds of House Republicans voted against certification of the results. Had Republican leaders not encouraged election denialism, the “stop the steal” movement might have stalled, and thousands of Trump supporters might not have violently stormed the Capitol in an effort to overturn the election.Second, after Mr. Trump was impeached by the House of Representatives for the Jan. 6, 2021, insurrection, Senate Republicans overwhelmingly voted to acquit him, even though many conceded that, in Senator Mitch McConnell’s words, the president was “practically and morally responsible” for the attack. The acquittal allowed Mr. Trump to continue his political career despite having tried to block the peaceful transfer of power. Had he been convicted in the Senate, he would have been legally barred from running again for president. In other words, Republican senators had a clear opportunity to ensure that an openly antidemocratic figure would never again occupy the White House — and 43 of them, including Mr. McConnell, declined to take it.Third, Republican leaders could have worked with Democrats to create an independent commission to investigate the Jan. 6 uprising. Had both parties joined forces to seek accountability for the insurrection, the day’s events would have gone down in U.S. history (and would likely have been accepted by a larger majority of Americans) as a criminal assault on our democracy that should never again be allowed to occur, much like Spain’s 1981 coup attempt. Republican leaders’ refusal to support an independent investigation shattered any possible consensus around Jan. 6, making it far less likely that Americans will develop a shared belief that such events are beyond the pale.Finally, with remarkably few exceptions, Republican leaders say they will still support Mr. Trump even if he is convicted of plotting to overturn an election. Alternatives exist. The Republican National Committee could declare that the party will not nominate an individual who poses a threat to democracy or has been indicted on serious criminal charges. Or Republican leaders could jointly declare that, for the sake of democracy, they will endorse Mr. Biden if Mr. Trump is the Republican nominee. Such a move would, of course, destroy the party’s chances in 2024. But by keeping Mr. Trump out of the White House, it would help protect our democracy.If Republican leaders continue to endorse Mr. Trump, they will normalize him yet again, telling Americans that he is, at the end of the day, an acceptable choice. The 2024 race will become another ordinary red vs. blue election, much like 2016. And as in 2016, Mr. Trump could win.Republican leaders’ acquiescence to Mr. Trump’s authoritarianism is neither inevitable nor unavoidable. It is a choice.Less than a year ago in Brazil, right-wing politicians chose a different path. President Jair Bolsonaro, who was elected in 2018, was an extreme-right politician who had praised torture, death squads and political assassination. Like Mr. Trump in 2020, Mr. Bolsonaro faced an uphill re-election battle in 2022. And like Mr. Trump, he tried to undermine public trust in the electoral system, attacking it as rigged and seeking to replace the country’s sophisticated electronic voting system with a paper ballot system that was more prone to fraud. And despite some dirty tricks on Election Day (police roadblocks impeded voter access to the polls in opposition strongholds in the northeast), Mr. Bolsonaro, like Mr. Trump, narrowly lost.But the similarities end there. Whereas most Republican leaders refused to recognize Mr. Biden’s victory, most of Mr. Bolsonaro’s major political allies, including the president of Congress and the newly elected governors of powerful states like São Paulo and Minas Gerais, unambiguously accepted his defeat at the hands of Lula da Silva, the winner on election night. Although Mr. Bolsonaro himself remained silent, almost no major Brazilian politician questioned the election results.Likewise, on Jan. 8, 2023, when angry Bolsonaro supporters, seeking to provoke a coup, stormed Congress, the office of the presidency and the Supreme Court building in Brasília, conservative politicians forcefully condemned the violence. In fact, several of them led the push for a congressional investigation into the insurrection. And when the Superior Electoral Court barred Mr. Bolsonaro from seeking public office until 2030 (for abusing his political power, spreading disinformation and making baseless accusations of fraud), the response among right-wing politicians was muted. Although the electoral court’s ruling was controversial, few Brazilian politicians have attacked the legitimacy of the court or defended Mr. Bolsonaro as a victim of political persecution.Not only is Mr. Bolsonaro barred from running for president in the next election, he is politically isolated. For U.S. Republicans, then, Brazil offers a model.Many mainstream politicians who preside over a democracy’s collapse are not authoritarians committed to overthrowing the system; they are careerists who are simply trying to get ahead. They are less opposed to democracy than indifferent to it. Careerism is a normal part of politics. But when democracy is at stake, choosing political ambition over its defense can be lethal.Mr. McConnell, House Speaker Kevin McCarthy and other top Republican leaders are not trying to kill democracy, but they have subordinated its defense to their own personal and partisan interests. Such reckless indifference could make them indispensable partners in democracy’s demise. They risk joining the long line of semi-loyal politicians littering the histories of interwar Europe and Cold War Latin America who sacrificed democracy on the altar of political expediency. American voters must hold them to account.Steven Levitsky and Daniel Ziblatt (@dziblatt), professors of government at Harvard, are the authors of “The Tyranny of the Minority” and “How Democracies Die.”The Times is committed to publishing a diversity of letters to the editor. We’d like to hear what you think about this or any of our articles. Here are some tips. And here’s our email: letters@nytimes.com.Follow The New York Times Opinion section on Facebook, Twitter (@NYTopinion) and Instagram. More