"I will not be voting for Donald Trump for president," she wrote in a Washington Post op-ed
published Monday night. "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."
She joins Sen. Mark Kirk, an Illinois Republican senator in a tough re-election race, in not voting for Trump. Kirk withdrew his endorsement of the GOP nominee in June.
In the op-ed, the moderate Mainer known for working across the aisle listed many instances when the Republican nominee made moves that were criticized by the media and his own party.
"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," she said.
However, there were three incidences in particular that led her to make her decision: when Trump mocked a disabled reporter
; when he said Judge Gonzalo Curiel
could not be fair about Trump University because of his Mexican heritage; and when he attacked a Gold Star family
, the Khans, after they spoke out against him at the Democratic National Convention.
"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," Collins wrote in the op-ed.
Her stance comes 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."
Collins said she will continue to honor Republican values even though she won't vote for Trump.
"It is because of Mr. Trump's inability and unwillingness to honor that legacy that I am unable to support his candidacy," she wrote."
Collins said she doesn't support either "major party nominee," but didn't say if she votes in November who she might support.