Now that Donald Trump has been defanged, leading Republicans are rushing to denounce him. It’s a little late. The circumstances were different then, but a year ago, only one Republican senator, Mitt Romney, backed impeachment. In a party that has been largely servile, Mr. Romney’s courage stands out.

Why, in the face of immense pressure, did Mr. Romney defend the rule of law? And what would it take to produce more senators like him? These questions are crucial if America’s constitutional system, which has been exposed as shockingly fragile, is to survive. The answer may be surprising: To get more courageous senators, Americans should elect more who are near the end of their political careers.

This doesn’t just mean old politicians — today’s average senator is, after all, over 60. It means senators with the stature to stand alone.

As a septuagenarian who entered the Senate after serving as his party’s presidential nominee, Mr. Romney contrasts sharply with up-and-comers like Josh Hawley and Ted Cruz, who seem to view the institution as little more than a steppingstone to the White House. But historically, senators like Mr. Romney who have reached a stage of life where popularity matters less and legacy matters more have often proved better able to defy public pressure.

In 1956, Senator John F. Kennedy — despite himself skipping a vote two years earlier on censuring the demagogue Joseph McCarthy — chronicled senators who represented “profiles in courage.” Among his examples were two legendary Southerners, Thomas Hart Benton and Sam Houston, who a century earlier had become pariahs for opposing the drive toward secession.

Benton, who had joined the Senate when Missouri became a state, had by 1851 been serving in that role for an unprecedented 30 years. Benton’s commitment to the Union led him to be repudiated by his state party, stripped of most of his committee assignments, defeated for re-election and almost assassinated. In his last statement to his constituents, he wrote, “I despise the bubble popularity that is won without merit and lost without crime.”

Houston enjoyed similar renown in his home state, Texas. He had served as commander in chief of the army that won independence from Mexico, and as the first president of the Republic of Texas. In 1854, he became the only Southern Democratic senator to oppose the Kansas-Nebraska Act, which he feared might break the country apart over the expansion of slavery. He did so “in spite of all the intimidations, or threats, or discountenances that may be thrown upon me,” which included being denounced by his state’s legislature, and later almost shot. Houston called it “the most unpopular vote I ever gave” but also “the wisest and most patriotic.”

It’s easy to see the parallels with Mr. Romney. Asked in 2019 why he was behaving differently from other Republican senators, he responded, “Because I’m old and have done other things.” His Democratic colleague Chris Murphy noted that Mr. Romney was no longer “hoping to be president someday.”

Nor was John McCain, one of the few other Republican senators to meaningfully challenge President Trump. By contrast, Mr. Hawley and Mr. Cruz — desperate to curry favor with Mr. Trump’s base — led the effort to challenge the results of last fall’s election.

Not every Republican senator nearing retirement exhibited Mr. Romney or Mr. McCain’s bravery. Lamar Alexander of Tennessee, an octogenarian former presidential candidate himself, voted not only against impeaching Mr. Trump last January, but against even subpoenaing witnesses.

Courage cannot be explained by a single variable. Politicians whose communities have suffered disproportionately from government tyranny may show disproportionate bravery in opposing it. Mr. Romney, like the Arizona Republican Jeff Flake — whose opposition to Mr. Trump likely ended his senatorial career — belongs to the Church of Jesus Christ of Latter-day Saints, which was once persecuted on American soil. In the fevered days after Sept. 11, the only member of Congress to oppose authorizing the “war on terror” was a Black woman, Barbara Lee.

But during that era, too, ambition undermined political courage, and stature fortified it. Virtually every Democratic senator who went on to run for president in 2004 — John Kerry, John Edwards, Hillary Clinton and Joe Lieberman — voted for the Iraq war.

By contrast, Mr. Kerry’s Massachusetts colleague, Ted Kennedy, who had been elected to the Senate in 1962, voted against it. The most dogged opposition came from a man who had entered the Senate three years before that, Robert Byrd of West Virginia. Despite hailing from a state George W. Bush had won, and seeing his junior colleague support the war, the 84-year-old Mr. Byrd, a former majority leader, tried to prevent the Senate from voting during the heat of a midterm campaign. His effort failed by a vote of 95 to 1.

If Americans want our constitutional system to withstand the next authoritarian attack, we should look for men and women like Senators Romney, Benton and Byrd, who worry more about how they will be judged by history than by their peers. George W. Bush was a terrible president — but might have proved a useful post-presidential senator because he would have been less cowed than his colleagues by Mr. Trump.

John Quincy Adams served in Congress for 17 years after leaving the White House. Given how vulnerable America’s governing institutions are, maybe Barack Obama could be convinced to do something similar.

Like most people, I’d prefer senators who do what I think is right. But I’d take comfort if more at least did what they think is right. That’s more likely when you’ve reached a phase of life when the prospect of losing an election — or being screamed at in an airport — no longer seems so important. America needs more senators who can say — as Daniel Webster did to his constituents in Massachusetts — “I should indeed like to please you; but I prefer to save you, whatever be your attitude toward me.”

Peter Beinart (@PeterBeinart) is professor of journalism and political science at the Newmark Graduate School of Journalism at the City University of New York. He is also editor at large of Jewish Currents and writes The Beinart Notebook, a weekly newsletter.

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