Sen. Susan Collins (R-Maine) on Monday said she won’t back GOP presidential nominee Donald Trump, joining a handful of Republican senators who say they cannot support Trump because of his rhetoric and temperament.

“Donald Trump does not reflect historical Republican values, nor the inclusive approach to governing that is critical to healing the divisions in our country,” Collins wrote in a Washington Post op-ed.

In one of the strongest declarations against Trump by a sitting Republican lawmaker, Collins cited Trump’s “constant stream of cruel comments and his inability to admit error or apologize” as evidence of his “unsuitability for office.” She said Trump’s propensity to denigrate critics and his “disregard for the precept of treating others with respect” give her particular pause.

“Mr. Trump lacks the temperament, self-discipline and judgment required to be president,” Collins wrote.

For months, Collins, a moderate Republican, made her reservations about Trump very clear, and even suggested she would be open to voting for Democratic nominee Hillary Clinton.

“I’m not going to say never, because this has been such an unpredictable situation, to say the least,” Collins told The New Yorker’s Ryan Lizza in June.

Collins on Monday revealed that her final straw was Trump’s attacks on Khizr and Ghazala Khan, the parents of a slain Muslim-American war hero, which she called “inconceivable.”

Like many Republicans torn between Trump’s toxicity and their commitment to their party, Collins said she had held out hope that the GOP nominee would moderate his rhetoric and demeanor. She said his repeated attacks convinced her otherwise.

“I had hoped that we would see a ‘new’ Donald Trump as a general-election candidate — one who would focus on jobs and the economy, tone down his rhetoric, develop more thoughtful policies and, yes, apologize for ill-tempered rants,” she wrote.

“But the unpleasant reality that I have had to accept is that there will be no ‘new’ Donald Trump, just the same candidate who will slash and burn and trample anything and anyone he perceives as being in his way or an easy scapegoat. Regrettably, his essential character appears to be fixed, and he seems incapable of change or growth.”

Read Collins’ full op-ed here.