Mitt Romney has said he will vote to remove Donald Trump from office as the president's impeachment trial draws to a close.
He is the only Republican senator to say he will convict Mr Trump of abuse of office ahead of a vote on two articles of impeachments.
But his vote is not expected to affect the outcome of the trial, with the president set to be cleared on Wednesday.
The first vote is on whether Mr Trump abused power by putting pressure on Ukraine and holding back millions of dollars of military aid.
The second is about whether he obstructed Congress refused to allow staff to testify at earlier impeachment hearings.
“I believe that attempting to corrupt an election to maintain power is about as egregious an assault on the Constitution as can be made,” Romney told the New York Times.
“And for that reason, it is a high crime and misdemeanour, and I have no choice under the oath that I took but to express that conclusion.”
Romney has at times been out of step with his party during Mr Trump's impeachment trial.
He sided with Democrats in calling for more witness testimony, a move Republicans blocked.
Mr Trump needs a two-thirds majority of 67 votes to be ousted from his position.
However as there are 53 Republicans and 47 Democrats in the Senate, the president is expected to be cleared.
