GOP Sen. Susan Collins joins a list of prominent Republicans who will not be supporting their party's presidential nominee this fall.

"I will not be voting for Donald Trump for president," the Maine senator wrote in a Washington Post op-ed published Monday evening. "This is not a decision I make lightly, for I am a lifelong Republican. But 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, a fourth-term senator who is considered a moderate, has been critical of Trump throughout his campaign.

In the Post, she cites several examples of questionable behavior that led her "to the inescapable conclusion that Mr. Trump lacks the temperament, self-discipline and judgment required to be president." This includes his comments that a federal judge's Mexican heritage would prevent him from being impartial, Trump's criticism of the Khans, a Gold Star family, after they appeared onstage at the Democratic National Convention in July and criticized him, and his mocking of a disabled New York Times reporter.



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"With the passage of time, I have become increasingly dismayed by his constant stream of cruel comments and his inability to admit error or apologize," Collins wrote.

"I am also deeply concerned that Mr. Trump's lack of self-restraint and his barrage of ill-informed comments would make an already perilous world even more so," she added.

Collins isn't the first Republican senator to withhold support from the bombastic billionaire. Illinois' Sen. Mark Kirk also said he wouldn't be voting for Trump and withdrew his endorsement of the GOP nominee in June.

Her announcement comes on the same day 50 GOP national security experts signed a letter stating their opposition to Trump, saying he "would be the most reckless president in American history."

"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," wrote Collins. "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."

She wrote that she doesn't support either major-party nominee, and it's not clear who will get her vote come November.