"Donald Trump, in my judgment, would make a perilous world even more dangerous," Collins told CNN's Jamie Gangel. "I worry that his tendency to lash out and his ill-informed comments would cause dangerous events to escalate and possibly spin out of control at a time when our world is beset with conflicts. That is a real problem."
The moderate Republican senator said for her the "tipping point" in opposing Trump came over his feud with the parents of Capt. Humayun Khan, who was killed in the Iraq War. His father, Khizr Khan, harshly criticized Trump at a speech at the Democratic convention and Trump's first reactions were to question why Khan's wife, who was overcome with grief, didn't speak.
"Those are the parents who lost their son in the war in Iraq, when he showed absolutely no empathy or compassion for their terrible loss and instead attacked these two Gold Star parents and also attacked their religion, that was just the final straw for me," Collins said.
The break with Trump was a long time coming for the senator. Collins has spoken out against Trump for a number of his past statements, including when he criticized the service of Arizona Sen. John McCain, who was a POW in Vietnam.
She said that ultimately, she came to the decision that Trump is unfit to be president.
"I concluded that Donald Trump lacked the temperament, the judgment and the self-discipline to heal the divisions in our country -- which are very real -- and to be commander in chief," Collins said.
She also hit Trump for suggesting he may not honor NATO treaty commitments, saying it "embolden(s)" America's enemies around the world to be more aggressive. And she said his actions on the trail have disturbed her.
"I'm also very concerned that Donald Trump kept appealing to the worst instincts rather than the best part of the American people -- that he was inflaming prejudices, looking for scapegoats, and worsening the divisions that are in our country," Collins told CNN. "In the end, I felt that I just could not support a person with those qualities and a person who never seemed to learn from his mistakes and who seemed to be incapable of recanting his ill-chosen words."
Collins' comments came the day after she became the latest Republican, and one of the most senior sitting politicians, to break with her party's nominee, in a Washington Post op-ed. In her first interview explaining the decision, Collins, the senior senator from Maine, told Gangel she won't support Hillary Clinton, either.
Collins said she had concerns about Clinton's policies, which she said would bankrupt the nation, and her use of a private email server as secretary of state. Collins said she could vote for Libertarian Gary Johnson, although she has issues with his positions on drugs, or write in a candidate.
The senator is just the latest Republican to speak out against Trump, part of growing establishment frustration with the nominee. She is one of the most prominent officials to do so, breaking with Senate Majority Leader Mitch McConnell. But she is joined by many in the orbit of the Bush family, some vulnerable incumbents up for reelection, and dozens of former GOP national security officials and other former leaders who are concerned about Trump's policies and temperament.
It also comes as Trump struggles in the polls, following a substantial post-convention bounce for Clinton.
Collins says she has never not voted for her party's nominee, but defended herself against accusations of betrayal.
"I would say to (critics) that Donald Trump does not represent the values or the heritage of the Republican Party, and we should not be afraid to say that, and indeed, there is a risk to the Republican Party if Donald Trump is perceived as embodying our values," Collins said. "We are an inclusive party. We are not a party of prejudice. We are not a party that wants to sow the seeds of division. We need to heal this country."
Trump has yet to speak directly on Collins, though he tweeted
Tuesday morning, "I am running against the Washington insiders, just like I did in the Republican Primaries. These are the people that have made U.S. a mess!"
Collins said she was unfazed by any backlash, and brushed off the idea that she has lost touch with her constituents in Maine.
"I'm waiting to see what name he eventually calls me, but maybe he's being held back by his advisers," Collins said. "But I'm doing what I think is right."