Sen. Mitt Romney (R-UT), the Republican Party’s 2012 presidential nominee, just became the sole exception to the Senate’s party-line vote to acquit President Trump — and the first senator who has ever voted to remove a president of his own party.

Romney says he broke party lines to vote to convict on one of two charges, abuse of power, because he believes Trump’s conduct shows that the president is unfit and that he “should be removed from office.”

“I believe that the act [Trump] took — an effort to corrupt an election,” Romney told Fox News’s Chris Wallace, “is as destructive an attack on the oath of office and on our Constitution as I can imagine.”

Romney’s vote makes him a unique figure in America history.

Only two other presidents besides Trump have faced an impeachment trial: President Andrew Johnson in 1868, and President Bill Clinton in 1999. Both Johnson and Clinton were Democratic presidents, and no Democrat voted to convict either man (though some Republicans did vote to acquit).

Johnson was impeached on three charges and nearly convicted — 35 of the 36 senators necessary to remove him from office voted to do so. At the time, the Democratic Party was a rump party, virtually powerless in Congress because they were closely associated with the Southern white secessionists who waged war against the Union.

Only nine Senate seats were held by Democrats during the Johnson impeachment, and all nine voted to keep Johnson in office.

Similarly, President Clinton was charged with two offenses and acquitted on both — the Senate voted 55-45 to acquit him of a perjury charge, and 50-50 to acquit him of obstruction of justice. Every senator who voted to remove Clinton was a Republican.

It’s worth noting that Trump’s supporters, including his own legal team, have tried to discredit the impeachment effort by labeling it a “partisan impeachment.” But, until now, every impeachment trial of a president ended with the president’s fellow partisans lining up for acquittal.

Romney’s vote makes Trump’s impeachment trial the first bipartisan vote to remove a president in American history.