American history is a misleading metric by which to measure things. The 240-odd years that the United States has existed is a long time in human terms but a relatively short one when measured in presidencies. When there have been only 45 presidents, assessing what’s usual for a president demands some sort of parenthetical about the relatively small population to which the assessment applies.

That holds even more so for presidential impeachments, of which there have been three. There was a great deal of discussion during the impeachment inquiry and trial centered on President Trump of past precedent, but when we’re talking about two prior occasions — the impeachment trials of Andrew Johnson in 1868 and Bill Clinton in 1999 — past practice doesn’t mean a whole lot.

Nonetheless, on Wednesday, something significant even within that limited context occurred. Sen. Mitt Romney (R-Utah) announced that he would vote to convict Trump on the charge of abuse of power. In so doing, he would become the first member of a president’s party to vote to convict that president, out of 223 opportunities for senators to have done so.

In 1868, Johnson, a Democrat, faced 11 articles of impeachment from a Republican-controlled House. The Senate considered those articles, ultimately deciding that all but eight were “objectionable.” A vote was held on three, all centered on Johnson’s having violated the Tenure of Office Act that Congress passed to try to constrain his behavior in office.

Thirty-six votes were needed to convict Johnson and remove him from office in the then-smaller Senate. On all three counts, 35 Republicans voted to convict. On all three, 10 Republicans and nine Democrats voted to acquit.

If any one of those nine Democrats had voted against Johnson on any one of those three votes, he would have been removed from office. None did.

In 1999, Clinton, a Democrat, faced two articles of impeachment of four originally introduced in the House. On a charge of perjury, 45 Republicans voted to convict Clinton while 10 Republicans and 45 Democrats voted to acquit. On the other article, focused on obstruction of justice, five Republicans switched from not guilty to guilty. It didn’t matter, though; Republicans were still well shy of the 67 votes needed to remove him from office.

Here again, though, Clinton’s party held strong. In two votes, 45 Democrats remained loyal to their party’s president.

Which brings us to Wednesday. We can only assume how most Republicans will vote. But it is clear how Romney will vote — and, in so doing, he will become the first senator to vote to oust his party’s president from office.

Also noteworthy: The total in favor of Trump’s acquittal is itself entirely partisan. Only Republicans voted Trump not guilty on the impeachment charges — the first time no members of the opposing party have voted with the president. The votes to convict will come from Democrats, independents and a Republican.

What’s particularly remarkable about Romney’s decision is the political environment in which it comes. Congress is much more polarized now than it was for the Johnson or Clinton impeachments. So is the electorate: Data from Gallup released on the day Trump’s impeachment trial began show that the gap in approval of Trump between the parties is wider than it has been for any president on record.

Romney has some space to act against that trend by virtue of the state he represents. No traditionally Republican state saw a bigger erosion of support for the Republican candidate in 2016 than did Utah, a function both of the state’s skepticism of Trump and the fact that Romney, a favored son, had been on the ballot four years prior.

The senator’s defection from Trump will be a source of intense annoyance to the president and other Republicans. Trump derived an enormous amount of political capital from describing the impeachment as purely partisan, a designation that necessarily ignored support for impeachment from Rep. Justin Amash (I-Mich.), whose opposition to Trump led him to leave the Republican Party in July.

Trump and his allies can no longer call the impeachment purely partisan, even with the Amash caveat. Yes, the history is limited, but on Wednesday, Romney nonetheless entered his name into the history books once again.

In doing so, Romney arguably made the impeachment of Trump the least partisan impeachment in history.