Is Sen. Mitt Romney (R-Utah) really the first senator in history to vote against his own party’s president in an impeachment trial? He is. But that’s only because President Richard M. Nixon resigned in 1974 when he learned how many Republican senators were prepared to remove him from office. Had Nixon hung on, Romney would have had plenty of company.

Here’s how we know.

What happened in 1974

Between July 27 and July 30, 1974, the House Judiciary Committee passed three articles of impeachment against Nixon. The committee overwhelmingly approved the first two articles, charging Nixon with obstruction of justice and abuse of power, by votes of 27 to 11 and 28 to 10.

Not only did all of the committee’s 21 Democrats vote for both articles, but more than one-third of its 17 Republicans voted aye as well. Six Republicans voted to impeach Nixon for obstruction of justice, and seven voted to impeach him for abuse of power. The committee agreed more narrowly to a third article charging the president with contempt of Congress, with just two Democrats and two Republicans crossing party lines.

Compare that with 2019, when none of the 17 Republicans on the House Judiciary Committee voted to impeach President Trump on similar charges.

Republican defections on the House Judiciary Committee were a dark omen for Nixon in 1974. Nixon already suspected he would be impeached by the full House, where Democrats held a majority. But the GOP defections suggested that Nixon might also be convicted in the Senate, where the Constitution requires a two-thirds majority.

Democrats held 56 seats in the Senate, but 67 votes would be required to convict Nixon and remove him from office. That meant at least 11 Republicans, or a quarter of the Republican caucus, had to be willing to vote against Nixon. Since one-third of House Judiciary Committee Republicans had already defected, Nixon could easily lose a quarter of Senate Republicans.

What might have happened in 1974

In 1981, political scientists Brian R. Fry and John S. Stolarek tried to estimate how Congress would have voted on Nixon’s impeachment. The authors first showed that votes on the House Judiciary Committee were 85 percent predictable from each member’s party, ideological leaning and the popularity of the president in their district. Using those three variables, the authors predicted the votes of every other member of the House and Senate.

I have revised their 1981 analysis using modern, “Bayesian” methods. The Bayesian approach, made popular in political forecasting by Nate Silver, allows for uncertainty in predictions and simulates the probabilities of a range of possible impeachment votes.

According to my simulation, the House had about a 95 percent chance of passing the first two articles of impeachment, most likely with about a two-thirds majority. The third article, by contrast, had only a one-third chance of passing the House. Most likely it would have failed with a little under half the vote.

In the Senate, the president faced a fight but had reason to hope. According to my simulation, Nixon had about a 55 percent chance of conviction for obstruction of justice, a 40 percent chance of conviction for abuse of power and almost no chance of conviction for contempt of Congress. Overall, he had about a one-third chance of being acquitted on all three charges. While a one-third chance is nothing to be proud of, at the time of the committee vote, it might have provided Nixon some reason to hold on.

Then came the smoking gun

Within a week Nixon’s situation got much worse. On Aug. 5, in response to a Supreme Court order, Nixon released 64 tape recordings. One recording, immediately dubbed the “smoking gun,” contradicted Nixon’s claims that he had not known about or tried to cover up his campaign’s connection to the Watergate break-in. On the smoking gun tape, recorded less than a week after the burglary, the president and his chief of staff discussed the FBI’s Watergate investigation, suggesting that the CIA should get the FBI to drop it by claiming that doing so was in the interest of national security. That claim was bogus.

After that, all 11 of the president’s GOP supporters on the House Judiciary Committee recanted. All said they would vote to impeach Nixon, at least on obstruction of justice, when the question came before the full House.

After the most loyal Republican committee members jumped ship, the president didn’t need Nate Silver to know he was doomed. On Aug. 7, three leading Republicans — the House and Senate minority leaders and the previous Republican presidential candidate, Sen. Barry Goldwater (R-Ariz.) — visited the White House. On the “verge of tears,” Goldwater told Nixon that 85 senators would vote to convict him for obstruction of justice — implying about 30 GOP defections.

If a comparable meeting were held today, Senate Majority Leader Mitch McConnell (R-Ky.), House Minority Leader Kevin McCarthy (R-Calif.) and Romney would have visited Trump to tell him his presidency was over. Of course, that didn’t happen. Romney voted to remove the president from office, but no other Republican joined him. Certainly not McConnell.

A more loyal GOP today

Congress was markedly less partisan in 1974 than it is today. But according to historian Mark Nevin, partisan politics still played a major role in Nixon’s removal. Shortly after the Senate Watergate hearings began in May 1973, Nixon’s approval rating fell to 25 percent and never recovered. The president’s unpopularity dragged down the whole Republican Party, which lost four out of five special elections in 1974. Pollster George Gallup predicted that after the midterm election in November 1974, the GOP would hold fewer than one-third of the seats in Congress. He was right about the House, but too pessimistic about the Senate.

By contrast, Trump’s approval ratings, though never high, have held steady in the low 40s since before the impeachment inquiry began in September 2019. Republican members of Congress have had little political incentive to defect, and the only Republican who did defect, Romney, is paying a significant price within his party — although not necessarily in his home state of Utah. Still, history may judge Romney more kindly.

Paul von Hippel (@paulvonhippel) is associate professor of public affairs at the University of Texas at Austin.