In a Washington Post op-ed published Monday night, Sen. Susan Collins (R-Maine) cited three specific instances that convinced her she could not support Donald Trump, her party's presidential nominee.

"This is not a decision I make lightly, for I am a lifelong Republican," she wrote. "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." The "denigrating comments" he made about Sen. John McCain (R-Ariz.) and Fox News host Megyn Kelly bothered her, Collins explained, and she has been "increasingly dismayed by his constant stream of cruel comments and his inability to admit error or apologize." Collins was especially disappointed that "his attacks" were "directed at people who could not respond on an equal footing, either because they do not share his power or stature or because professional responsibility precluded them from engaging at such a level."

The three incidents that led Collins "to the inescapable conclusion that Mr. Trump lacks the temperament, self-discipline, and judgment required to be president" were his "mocking of a reporter with disabilities," "criticism of the grieving parents of Army Captain Humayun Khan," and "repeated insistence" that Judge Gonzalo Curiel, born and raised in Indiana, "could not rule fairly in a case involving Trump University because of his Mexican heritage." Collins wrote she had been hopeful Trump would change once he became the general election candidate, but she has "had to accept 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."

Being a Republican is "part of what defines me as a person," Collins wrote, and while she did not say who she will support in the election, she did reveal it won't be "either of the major party nominees." Read the entire op-ed at The Washington Post. Catherine Garcia