After any major national disaster or failure of government, it’s essential to study what happened and why, if for no other reason than to enact laws and policies aimed at preventing the same thing from happening again. From the Warren Report on the assassination of President John F. Kennedy to the Church Committee in the wake of the Watergate scandal, from the commission on the Sept. 11 attacks to the commission on the Deepwater Horizon oil spill, a thorough official reckoning makes for good government.

What could accountability look like in 2021? How does American democracy confront the scale of the damage wrought by the departing president — the brazen obliteration of norms, the abundant examples of criminal behavior, the repeated corruption and abuses of power by the highest officeholder in the land, even after he was impeached?

In short, how does America prevent the next Trump administration if it can’t properly hold the current one to account?

This is the task facing the country and the next administration, as Joe Biden prepares to assume the presidency after running, and winning, on a platform of national unity and healing. Does restoring the soul of America require an exorcism of the past four years? Or would that only deepen the nation’s divisions, making it impossible to move forward?

In a country as polarized as the United States in 2020, making even incremental progress on pressing issues would be a win. We’ve urged Mr. Biden to champion an agenda based on decency. The first step is to dial down the culture wars wherever possible, then pursue a policy agenda where there is ample common ground. But Mr. Biden should also champion accountability after four years that tested the outer limits of what America’s democracy could handle.

His victory is itself a powerful form of accountability. A majority of American voters, given the chance to render their verdict on one term of President Trump, rejected his bid for another. Yet defeating Mr. Trump at the polls was only the first step toward recalibrating the country’s moral compass. Two more challenges remain: The first, determining how to investigate the past administration. The second, determining how to ensure that subsequent presidents face more formidable obstacles to wrongdoing than Mr. Trump faced.

Any honest accounting of the past four years needs to begin by establishing a shared set of facts about what happened. There is ample evidence already that Mr. Trump and some of his top allies may have broken multiple federal laws by committing campaign-finance violations, lying to federal investigators and obstructing justice, to name a few. Even if he is not prosecuted by federal authorities, Mr. Trump and his businesses face at least two separate tax-fraud investigations in New York. Many of Mr. Trump’s associates have already been convicted of various crimes.

Yet there are still many lingering questions, foremost among them: Did the president’s business interests influence his conduct of foreign and domestic policy? The American people have a right to know if that was so.

There are also powerful arguments against an administration investigating and prosecuting its opponent. No matter how strong the evidence or how independent Mr. Biden’s attorney general, it will inevitably look to half the country like a political hit job. Perhaps that shouldn’t matter, but from a practical standpoint it’s hard to convince Americans that the Justice Department is not politicized when it’s prosecuting the preceding administration.

Then there’s the issue of triage. The moment Mr. Biden takes office he will be saddled with a staggering pileup of emergencies demanding his immediate attention: a pandemic killing thousands of Americans daily; a crippled economy, mass unemployment and miles-long food lines; a worsening climate crisis; and the growing threat of right-wing terrorism and racist violence. Then there was this week’s revelation that the Russian government is behind a huge, monthslong cyberattack on dozens of federal agencies and companies, posing what officials called a “grave risk” to the federal government.

Despite the challenges, the nation has to move forward.

Many of the most needed reforms fall into the same category as those that were adopted in the aftermath of Watergate: reducing the power of the presidency and re-empowering institutions, like Congress, that are supposed to serve as checks on an imperious executive. Those reforms managed to hold the line for a while, but they turned out to be ineffective at reining in a president with Mr. Trump’s sheer tenacity and disregard for the rule of law. While Mr. Trump failed to fully exploit their weaknesses, a more devious and competent demagogue would be much more likely to succeed.

Corruption and abuse of power are the most urgent issues in need of addressing.

Four years into Mr. Trump’s presidency and nearly five years since he promised to release his full tax returns, the American people still don’t know how much his personal financial interests and entanglements are intertwined with his administration’s domestic- and foreign-policy decisions. He has an affection for strongmen, but is his solicitousness toward leaders like Russia’s Vladimir Putin, Turkey’s Recep Tayyip Erdogan and Saudi Arabia’s Mohammed bin Salman a result of something more mercenary? New laws compelling all presidential candidates to release at least 10 years of their tax returns, as well as a comprehensive list of any possible conflicts of interest, financial and otherwise, should be an obvious step toward reform. Legislation should also bar presidents from being involved in overseeing any business while serving in office.

These reforms would need to apply throughout the executive branch. As the nation has seen, Mr. Trump’s administration has been awash from the start in self-dealing, ethical investigations and scandals. From cabinet secretaries to agency heads, the list is long, and likely incomplete.

The second major area for reform involves presidential abuse of power, which includes everything from violations of the Hatch Act to the destruction of presidential records. The larger concern is the politicization of law enforcement. Mr. Trump was open in his belief that the Justice Department should do his bidding. He pressured his attorneys general, from Jeff Sessions through William Barr, to protect him and his allies and prosecute his perceived enemies. Sometimes they consented. Other times they resisted. But if law enforcement is to operate fairly and effectively, the American people have to see it as independent from politics.

Mr. Trump has also abused his power by pardoning friends and associates who have been convicted of serious crimes. More recently he has floated granting pre-emptive pardons to family members and even himself, which may be unconstitutional. Mr. Trump isn’t the first to test the pardon power’s limits, but he has politicized and personalized it to an unparalleled degree. Since the power is essentially absolute, any meaningful change to it would require a constitutional amendment.

As for the other reforms, some can be accomplished through executive orders, more stringent regulations or internal agency memos. But the most lasting will have to be enacted through federal laws, like the Protecting Our Democracy Act, a bill Democrats in the House of Representatives introduced in September. The legislation includes many measures that Republicans have supported in the past. Among other things, it would prohibit self-pardons, give Congress more power to enforce subpoenas, reduce the chance of politically motivated prosecutions by requiring more transparency from the Justice Department and provide more protection for inspectors general and whistle-blowers.

The problem with passing laws is that you need both houses of Congress, and to date many Republican lawmakers have showed little to no interest in addressing these sorts of abuses. Most of them still refuse to acknowledge Mr. Biden’s victory. Perhaps once they do, the prospect of similar abuses by a Democratic president might give them the incentive they need to get on board with reforms.

In the absence of any cooperation from the Senate, Mr. Biden could establish a bipartisan executive commission. Its primary task would be to tell the story of what happened and to propose remedies. It would need to have the power to compel the production of both witnesses and documents, and the mandate to produce as complete and accurate a record as possible of the violations of laws and norms by the Trump administration.

It’s been four years since Americans lived under a president who placed the country’s well-being and security above his own personal interests. Simply by occupying the Oval Office, Mr. Biden can begin to repair the damage caused by his predecessor. With every act he takes, he will send an important message to the American people, and the world, about what a president should do — and, perhaps more important, what a president should not do, even if he technically has the power to do it.

That’s why Mr. Biden’s victory is significant, apart from any specific reforms. “So much of whether these reforms will be successful and whether the prestige of the presidency and the dignity of the presidency will be respected is not going to depend on legal reform,” said Jack Goldsmith, a lawyer in the George W. Bush administration who co-wrote a book about reconstructing the presidency after Mr. Trump. “It’s going to depend on the identity of the person who’s the president. So not everything can be done by law. Some things need to be done by elections.”

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