Sen. Susan Collins, R-Maine, came out hard against Donald Trump Monday, and announced she will not vote for her party's presidential nominee this November.

"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 said in an op-ed published in the Washington Post.

The senator acknowledged that Trump had tapped into a voting bloc long ignored by the nation's lawmakers. She also said she understands the appeal of a candidate who openly fights the restrictions of political correctness.

However, she added, there's a difference between fighting politically correct culture, and simply eschewing common decency.

"Mr. Trump did not stop with shedding the stilted campaign dialogue that often frustrates voters. Instead, he opted for a constant stream of denigrating comments, including demeaning Sen. John McCain's (R-Ariz.) heroic military service and repeatedly insulting Fox News host Megyn Kelly," the Maine lawmaker wrote.

"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 added.

More distressing than these examples, she wrote, are Trump's attacks on people who do not have similar platforms with which they can respond to his attacks.

These attacks specifically, she concluded, have revealed Trump is "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," the senator wrote.

"Instead, he opts to mock the vulnerable and inflame prejudices by attacking ethnic and religious minorities," she added.

The worst examples of such behavior, she explained, are Trump's attacks on a reporter with disabilities, his ongoing fight with Judge Gonzalo Curiel and the GOP nominee's back-and-forth with a Gold Star family of Muslim faith.

"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. "It is reckless for a presidential candidate to publicly raise doubts about honoring treaty commitments with our allies. Mr. Trump's tendency to lash out when challenged further escalates the possibility of disputes spinning dangerously out of control."

She had hoped for the oft-discussed presidential "pivot." She had hoped he would grow more serious as Election Day drew closer.

Sadly, Collins concluded, what you see it what you get. The Trump that won the GOP presidential primary is the Trump Republicans get. There will be no pivot. There will be no moderating.

"[H]is essential character appears to be fixed, and he seems incapable of change or growth," she wrote.

"Some will say that as a Republican I have an obligation to support my party's nominee. I have thought long and hard about that, for being a Republican is part of what defines me as a person," she added. "I revere the history of my party, most particularly the value it has always placed on the worth and dignity of the individual, and I will continue to work across the country for Republican candidates. It is because of Mr. Trump's inability and unwillingness to honor that legacy that I am unable to support his candidacy."