Yet another Republican is publicly disowning Donald Trump

Senator Susan Collins from Maine became the latest senior Republican, and the most prominent so far, to break rank with her party by announcing she will not vote for Donald Trump for president. Her defection is of a heavier caliber than any other so far in the party, and may open the gates to more high-profile rejections.

"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 wrote in a critical op-ed in the Washington Post Monday disavowing her own party's presidential nominee.

Collins added that she could not bring herself to vote for Trump because of his fondness for bullying and "disregard for the precept of treating others with respect," which she pointed out was "an idea that should transcend politics."

Fellow Republican senators Ben Sasse of Nebraska and Mark Kirk of Illinois have said they would pretty much rather vote for a dalmatian than elect Donald Trump president. Others, such as Utah's Mike Lee and Arizona's Jeff Flake, have been deeply critical of Trump and refused to support him. Ted Cruz, who lost the Republican nomination to Trump in a brutal primary campaign, also did not endorse him during his speech at the Republican National Convention last month.

But Collins' disavowal of Trump carries a different type of weight. She is one of the few moderate — and female — Republicans left in Congress, who is known for compromising with her Democratic colleagues and is highly respected both in Washington and her home state. She is also senior member of the Senate's committee on Homeland Security and her critique adds to the growing sentiment in both parties that Trump poses a national security threat if he were to become president.

Collins' scathing op-ed came on the same day that 50 senior national security experts signed a letter declaring Trump was unfit to be president. And while senators may be saying that they aren't voting for Trump, none yet have said they're going for Hillary Clinton — but an increasing number of Republican donors are. Those include mega-donor and tech billionaire Meg Whitman, who said last week she would actively raise money for the Democratic nominee.

Also on Monday, former CIA operative Evan McMullin announced he is launching an independent run for president as a conservative alternative to Trump.

Collins' decision to endorse Clinton over Trump could be the tipping point many other like-minded Republicans were waiting for. Until now, the consensus among many Republicans has been that Trump might not be their guy but he is a better option than conceding the White House to Clinton. But that position could be changing after the past two weeks, which Trump has spent insulting pretty much everyone in and outside of his party.

It was Trump's "attacks directed at people who could not respond on an equal footing," Collins wrote, that made her realize he was "unworthy of being our president," she wrote. Specifically, she cited Trump's recent feud with the family of slain war hero Humayun Khan, his attacks on a federal judge because of his Mexican heritage, and his mocking of a disabled reporter as reasons he was temperamentally unfit to lead the country.

Collins had hoped Trump would pivot into a more normal candidate as the general election campaign got under way but, as she wrote, "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."

"Regrettably, his essential character appears to be fixed, and he seems incapable of change or growth," Collins added.

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