Story highlights She's a Republican who's been in the Senate since 1997

She says Trump lacks self-restraint

Washington (CNN) Susan Collins, a Republican who's been in the Senate since 1997, said she is not going to vote for Donald Trump as president, finding him unsuitable for office "based on his disregard for the precept of treating others with respect, an idea that should transcend politics."

"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.

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