With the House likely to vote to impeach President Trump, what electoral repercussions (if any) would House or Senate Republicans face from their votes on whether to impeach the president or remove him from office? If some members of the GOP break ranks, would their constituents reward or punish them at the polls? History offers a lesson.

The votes for or against impeaching President Richard Nixon in the House Judiciary Committee in 1974 were public votes in committee, cast after televised committee debate under an intense media spotlight.

Our research shows that in November 1974, voters severely punished GOP House members who voted against impeachment — but not those who voted in favor. The more recent impeachment of President Bill Clinton in 1998, on the other hand, offers no useful lessons for today, since congressional votes on impeachment were closely tied to members’ ideological leanings. Parties had already begun to polarize into what we see today.

Of course, the context in 1974 was different. Among other differences, most Americans then got their news from the same sources; today, they consume news through partisan outlets and social media feeds that frame what they hear quite differently.

Still, let’s take a closer look.

Here’s how we did our research

In July 1974, the House Judiciary Committee voted on articles of impeachment. Seven of the 17 Republican committee members joined every Democrat to vote to impeach Nixon. Ten Republicans on the committee stuck with the president and voted against impeachment. But Nixon’s guilt was clearly exposed in August when the infamous “smoking gun” tapes revealed the president discussing plans to obstruct the investigation with his chief of staff, H.R. Haldeman.

To measure voters’ reactions, we find the difference between each member’s share of the two-party vote in 1974, after the hearings; and 1972, the election before the hearings. We had to exclude seven members from our analysis for a few reasons. Five had their district boundaries changed, faced no opponent or retired before the 1974 election.

We also set aside impeachment opponent Trent Lott from Mississippi, who later became the GOP leader in the Senate, and Republican impeachment supporter William Cohen from Maine, who later served as Bill Clinton’s secretary of defense. Lott gained a whopping 16 percentage points from 1972 to 1974, an unusual surge against the Democratic tide of 1974. That’s because Mississippi at the time was making a swift transition from a Democratic to a Republican state in congressional politics.

Cohen also had a remarkable election, gaining an astounding 17 points. As an articulate advocate for the impeachment cause, Cohen stood out for his youthful vigor and liberal politics. For both Lott and Cohen, it would be hard to separate the effects of their impeachment stances from other reasons for their electoral success.

That leaves us with 10 GOP members to study. Four supported impeachment, and six did not. These Republicans were otherwise very much alike, virtually identical in ideological orientation, electoral history and district partisanship. In other words, with similar roll-call records and electoral constituencies, only their votes on impeaching Nixon clearly differentiate the two groups of Republican House members.

Nixon’s loyalists paid at the polls

So how different were the two groups’ fates in the 1974 election? The first thing to note was that the Watergate episode meant that 1974 was a very bad year for Republicans, who suffered a net loss in the House of 49 seats. It was difficult to swim against this anti-GOP current, but the Judiciary Committee members who cast their lot with impeachment survived fairly well. Of the four pro-impeachment members, only one lost his reelection bid. By contrast, voters sent four of the six anti-impeachment members packing.

On average, the six pro-Nixon (anti-impeachment) members lost 13 percentage points between 1972 and 1974. The four anti-Nixon members? These impeachment supporters lost 3.8 points — a more than nine-point difference. Granted, these are small numbers. But the difference is real. We can see that in the figure below, which shows the relative change in each member’s vote margin between the two elections. We set each member’s 1972 vote margin in 1972 to zero, and then plot the change in their vote margin in 1974.

The percentage change in vote margins for all six Nixon supporters declined more than did the vote margins for all but one of the four who voted for Nixon’s impeachment. The one exception was Harold Froehlich of Wisconsin, the only Republican on the Judiciary Committee who voted to impeach Nixon and lost in 1974, but barely.

Lessons for today’s GOP?

Some might dismiss the findings, given how few members’ elections we have to analyze. Others will wonder whether the results are relevant in a markedly more partisan climate of today’s electoral politics. And GOP members who vote to impeach Trump could face a primary challenger in 2020, possibly a bigger threat than risking a general election outcome by supporting Trump.

Of course, in 1974, the House Judiciary Committee agreed to articles of impeachment before the “smoking-gun tape” had been revealed. Nixon’s supporters could not anticipate his resignation in disgrace before Election Day. Will Trump’s standing be similar to Nixon’s when today’s House members run for reelection in 2020?

Robert S. Erikson is a professor of political science at Columbia University.

Gerald Wright is a professor of political science at Indiana University.

Together they are co-authors of “Statehouse Democracy” (Cambridge University Press, 1994).