Sen. Susan Collins, a Republican who has represented Maine for 20 years, said Monday she will not be voting for Donald Trump.
Collins explained her decision in a column in the Washington Post.
"This is not a decision I make lightly, for I am a lifelong Republican," she said. "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 said Trump was unsuitable for the presidency because of his inability to treat others with respect, a value that should go beyond party lines. Particularly alarming is his record of attacking people who lack his power or cannot respond because of professional responsibilities, she said, pointing to Trump's mocking of a reporter with a disability, his comments on a federal judge whose parents immigrated from Mexico, and his criticism of the parents of a fallen Muslim-American soldier.
"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," Collins 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."
Collins told CNN Tuesday that she didn't know who she was going to vote for, and that she was not going to support Hillary Clinton.
"I have a lot of concerns about Hillary Clinton and I am not going to support her," Collins said. "I may well end up writing in a name for president."
Collins added that she remains a Republican and will support other Republican candidates in their races around the country. In particular, she said she values the Republican tenet of individuals' worth and dignity.
"It is because of Mr. Trump's inability and unwillingness to honor that legacy that I am unable to support his candidacy," she said.
Collins' statement came after another anti-Trump Republican announced an independent presidential bid. Some Republican donors have stepped back their support of Trump in the last week, and on Monday, 50 former national security and foreign policy Republican leaders warned that Trump would be a "dangerous" and "reckless" president.