For more than 40 years, virtually every major scandal in American politics has been likened to Watergate. But no presidential deed — not Ronald Reagan's trading of arms for hostages in Iran-contra, not Bill Clinton's coverup of his affair with a young White House aide in the Monica Lewinsky affair — ever rivaled any of Richard Nixon's serial abuses of executive power in their gravity.

Until now.

President Trump's firing of FBI Director James B. Comey — who was overseeing the probe of the Trump campaign's possible collusion with Russia to influence the 2016 election — was technically legal, since the president acted within his official authority. But it plainly violates the democratic norms that have long governed the use of presidential power, and bears Nixonian overtones. With Trump mirroring Nixon's brazen highhandedness, the most pressing question is whether Republicans in Congress will muster the same courage and integrity Republicans did after Watergate.

Comey's unceremonious firing brings to mind the Saturday Night Massacre of October 1973, when Nixon ordered the sacking of Archibald Cox, the special prosecutor who was pursuing Watergate and who was demanding to hear the secret White House recordings that might contain evidence of Nixon's role in the scandal. On that fateful night, the top two Justice Department officials, Attorney General Elliot Richardson and his deputy, William Ruckelshaus, resigned rather than carry out Nixon's orders. Solicitor General Robert Bork finally fired Cox, and days later abolished the special prosecutor's office altogether.

Nixon's actions then were also technically legal. But as everyone could see, they constituted a blatant attempt to snuff out an investigation that was closing in on him. In that sense, the parallels with Trump's firing of Comey seem striking.

In both instances, the sitting president was suspected of having tampered with the machinery of our democratic presidential elections. In Watergate, many feared that Nixon had ordered or covered up his aides' burglary of the opposing party's headquarters; today, circumstantial evidence is mounting that Trump or his aides may have colluded with Russia to hack e-mail accounts to sway public opinion against his opponent, Hillary Clinton.

In both instances, too, postelection investigations had amassed strong evidence of serious wrongdoing. And in both instances, it was vital to guarantee that the president not be permitted to unilaterally end an inquiry into his own misdeeds.

In the parlous days of 1973, it fell to Congress to ensure that the system worked. Likewise, in the days ahead, Congress again will decide whether our nation's democratic norms are upheld or whether, under Trump, America takes a step toward the model of Russia, Turkey or Venezuela — countries where some trappings of democracy still remain but the rule of law and the will of the voters have come to mean little.

Nixon's rash decision to fire Cox backfired, in the first place, because the press and the public spoke out, pressuring their representatives to act. On NBC, a news anchor declared, "The country tonight is in the midst of what may be the most serious constitutional crisis in its history." The Republican-leaning Time magazine ran its first-ever editorial, urging the president to step down.

Record numbers of telegrams swamped Congress. Even GOP congressmen reported mail running 9 to 1 in favor of impeachment. House Democratic leaders quickly agreed to have the Judiciary Committee open an inquiry to see if Nixon had committed impeachable acts. Over the following days, House members introduced 44 Watergate-related bills, half of which called for impeachment proceedings or investigations and 12 of which demanded a new special prosecutor.

As Watergate historian Stanley Kutler wrote, "The time had come to watch congressmen's feet, not their mouths." Democrats controlled both chambers of Congress, making it easier to assign the Judiciary Committee subpoena powers, as it quickly did. But just as notably, a good many Republicans put country over party.

Prodded in many cases by their outraged or dispirited constituents, they broke with the president. They demanded that he appoint a new prosecutor (Leon Jaworski, who proved, against some expectations, to be just as tenacious as Cox), or that he turn over the secret White House tape recordings, or that he resign. Some assumed leading roles in the opposition, forging a bipartisan front in support of democracy and against one-man rule.

The question now is whether congressional Republicans will again rise to the occasion. Many congressmen and senators loudly criticized Trump during the campaign, but since he took office few have done much to stand in his way. Yet the time for evasions is over. Of all the authoritarian tendencies and bullying behaviors Trump displayed in 2016, the most disturbing was always his affinity for Vladimir Putin and his apparent unconcern with — or active encouragement of — Russian interference in our democracy. Comey's firing may well have ended any prospect of the truth emerging about Trump's relationship with the Russian autocrat. But if some prospect remains, it's the Republicans who must actively choose to keep it alive.

Will Republicans in Congress allow the intelligence committees to chase down, unfettered, the full story of the contacts between Russian officials and Trump aides including Michael Flynn, Roger Stone, Carter Page and Paul Manafort? Will senators acquiesce in letting Trump name a loyalist to run the FBI, someone who will pull the plug on its investigation — or will they insist on a genuinely independent replacement for Comey? Will they authorize subpoena powers to the key committees and pledge themselves to the vigorous pursuit of the truth?

The Tuesday Night Massacre is, surely, a moment of truth.

David Greenberg is a history professor at Rutgers University. He wrote this article for the Washington Post.