Sen. John McCain (R-Ariz.) speaks to reporters at the Capitol on May 10. (Joshua Roberts/Reuters)

Tuesday’s firing of FBI Director James B. Comey is an alarming development in the months-long story of whether Russia has colluded with, influenced or attempted to influence Donald Trump or his presidential campaign. While Comey has been rightly criticized for his actions during the 2016 election, we suspect that wasn’t the real reason he was dismissed. Whether Comey was fired because he and the Russia investigation were on television too much — “because of television coverage” is always a plausible reason for any Trump action — or because of something more sinister, the fact remains that the president fired the person in charge of an investigation involving the president. Whatever independence the investigation had has been gutted. As of Wednesday, only Republicans can restore it — and history suggests that they won’t.

Many — including the front pages of The Post and the New York Times — are drawing parallels between the firing of Comey and the “Saturday Night Massacre.” On that day in October 1973, Richard Nixon demanded the firing of Watergate independent prosecutor Archibald Cox. Attorney General Elliot Richardson and Deputy Attorney General William Ruckelshaus resigned rather than fire Cox. (Solicitor General Robert Bork carried out Nixon’s order.)

The most important lesson for the situation now facing the country is the reason Cox’s position was created: The opposition demanded it. In 1973, Democrats controlled both the House and Senate. When Nixon nominated Richardson to run the Justice Department in May 1973, Senate Democrats refused to confirm Richardson until he chose a special prosecutor. Democratic control of the legislative branch, in other words, was essential to creating the investigation that proved Nixon’s involvement and led to his resignation.

Democrats don’t have that leverage now. They can try to block Comey’s replacement or even grind the entire Senate to a halt unless Attorney General Jeff Sessions or Deputy Attorney General Rod J. Rosenstein appoint an independently-minded special prosecutor. (That may seem highly unlikely, but after Bork fired Cox, he picked Leon Jaworski, who saw the Watergate investigation through to Nixon’s resignation.) But without a majority, they will need at least three Republicans to publicly stand with them. At first glance, the odds of that may seem good. GOP senators such as Jeff Flake (Ariz.), John McCain (Ariz.) and Ben Sasse (Neb.) have all publicly questioned the firing. So has Intelligence Committee chair Richard Burr (N.C.).

History is not so encouraging. For months after the Watergate burglars were arrested in June 1972, Republicans continued to defend the president. Sen. Bob Dole described the first stories of The Post’s now-famous reporting as “a barrage of unfounded and unsubstantiated allegations.” Ronald Reagan described the burglars as “well-meaning individuals committed to the reelection of the president.” The Senate did vote unanimously to create the Senate Watergate Committee, but Republicans tried unsuccessfully to shift the committee’s focus to the 1964 and 1968 elections, and they harshly criticized the committee. Minority Leader Hugh Scott called the resolution “wild” and “unbelievable” while Ted Stevens predicted the committee would be “nothing but a political witch hunting body.” It wasn’t until a series of “smoking guns” emerged in criminal trials and Democrat-controlled hearings that some Republicans began to turn on the White House in earnest. Even then, many stayed loyal to Nixon: In July 1974, less than half of the Republicans on the House Judiciary Committee voted for the articles of impeachment. Given Republicans’ rhetoric at the time, it seems unlikely that had they been in the majority they would have supported a select committee or demanded a special prosecutor.

Recent history also justifies fears that Republicans will not stand up to Trump. Flake, McCain, Sasse and other senators have all clashed publicly with the president before. But those are just words, and talk is cheap. With the occasional exception when Republicans have been able to spare one or two votes, GOP senators have marched in lockstep with the Trump White House. McCain in particular has continued his years-long pattern of tut-tutting Republican leaders and then voting with his party anyway.

If Flake, McCain and others want to show us they are truly troubled, then they will need to do more than put out a statement. They need to join with Democrats and refuse to vote for a new FBI director (and perhaps even other Trump appointees or legislation) until a special prosecutor is appointed. Nothing short of that is acceptable.

All is not lost if Republicans continue to cower as they have. The power still lies with the American people to vote more Trump critics into office and make it clear that toadying to Trump is political suicide. But the 2018 midterms are 18 months away, with a tough Senate map for the Democrats, and 2020 is even farther. A few Republicans can save the country from a whole lot of trouble if, against the odds, they finally grow a backbone.