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Sen. Susan Collins of Maine will not be voting for Donald Trump.
The four-term senator became the latest Senate Republican to withhold support for Trump.
In a sharply critical op-ed for the Washington Post, the senator said that as a lifelong Republican she had not made her decision lightly. Others in the party expect her to fall in line, she wrote.
But, she said, after watching — and waiting — for a "new" Trump to emerge on the campaign trail, she had come to the conclusion that Trump is set in ways that do not reflect the values of her party and that she cannot support.
"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," Collins 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."
"My conclusion about Mr. Trump’s unsuitability for office is based on his disregard for the precept of treating others with respect, an idea that should transcend politics," she said.
"Instead, he opts to mock the vulnerable and inflame prejudices by attacking ethnic and religious minorities."
Collins also said that her concerns over Trump fed into broader worries about national security.
"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," she wrote.
Collins is known as one of the few remaining moderate Republicans, and she is not up for reelection this year.
She joins Republican Sens. Ben Sasse of Nebraska, Lindsey Graham of South Carolina, Jeff Flake of Arizona and Mark Steven Kirk of Illinois in opposing Trump.
See our interactive detailing the Trump support spectrum.