Republican Senator Susan Collins of Maine will not be voting for Donald Trump, she wrote in a Washington Post op-ed Monday night.
"This is not a decision I make lightly, for I am a lifelong Republican," Collins wrote.
She does not support Hillary Clinton either, she wrote.
Collins was disturbed by Trump's "cruel comments" throughout the primary campaign and "his inability to admit error or apologize," she wrote.
"But it was his attacks 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 — that revealed Mr. Trump as unworthy of being our president," Maine's senior senator wrote.
Collins singled out three incidents that turned her off from the nominee. First was Trump's mocking of a reporter with disabilities. Second was when he suggested that a federal judge's Mexican heritage made him biased against him. Third, Collins pointed to Trump's criticism of the family of a dead American soldier.
Collins had hoped that Trump would moderate his behavior during the general election.
"But the unpleasant reality that I have had to accept is that there will be no 'new' Donald Trump," Collins wrote.
She pledged to work for other GOP candidates around the country.
Several of her Senate colleagues — including Nebraska Sen. Ben Sasse, South Carolina Sen. Lindsey Graham and Illinois Sen. Mark Kirk — have also ruled out voting for Trump. Of that group only Kirk is up for re-election this year. Arizona Sen. Jeff Flake has repeatedly said he cannot back Trump yet.