The number of influential Republican officials saying that they can't vote for either Donald Trump or Hillary Clinton is growing as Sen. Susan Collins (R-Maine) pledges she won't vote for Trump. (Jenny Starrs/The Washington Post)

Sen. Susan Collins of Maine, a centrist Republican, announced Monday that she will not vote for Donald Trump, joining a growing list of GOP officials who have come out against the mogul for president.

Collins writes in a Washington Post op-ed published online Monday night that three incidents led her to conclude that the GOP nominee lacks essential qualifications to be president: his mocking of a disabled reporter, his intense criticism of a U.S.-born federal judge who is Mexican American and, most recently, his attacks against the Muslim American parents of an Army captain who was killed in Iraq.

“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,” Collins writes. "Instead, he opts to mock the vulnerable and inflame prejudices by attacking ethnic and religious minorities.”

The Maine senator does not say in the op-ed who she plans to vote for in November. But she says she does not support Hillary Clinton, writing, "As we have seen with the dissatisfaction with both major-party nominees — neither of whom I support — these passions are real and the public will demand action."

Sen. Sen. Susan Collins (R-Maine), left, announced Aug. 8 that she will not vote for Republican presidential nominee Donald Trump, right. (Photo of Collins by Mark Wilson/Getty Images; photo of Trump by Evan Vucci/AP)

In an interview with the New Yorker magazine published in June, Collins said it was possible that she would vote Clinton. “I worked very well with Hillary when she was my colleague in the Senate and when she was secretary of State,” she said at the time. But she added that "it is unlikely that I would choose to vote for the Democratic candidate."

Collins becomes the latest in a small but growing group of Republican senators who have said they will not vote for Trump. Sen. Ben Sasse of Nebraska has vocally opposed the mogul for months. Sen. Lindsey O. Graham of South Carolina told CNN in May that he did not intend vote for Trump or Clinton.

Others, like Sen. Jeff Flake of Arizona, have criticized Trump sharply, but stopped short of definitively saying they will not vote for him.

Fifty national security officials who served under Republican presidents signed a letter Monday saying Trump is “not qualified” to be president.

Collins writes that Trump’s criticism of Khizr Khan, who spoke out against the mogul at the Democratic National Convention and whose son Capt. Humayun Khan was killed in Iraq, was an especially bad moment for the GOP nominee.

“Rather than honoring their sacrifice and recognizing their pain, Mr. Trump disparaged the religion of the family of an American hero. And once again, he proved incapable of apologizing, of saying he was wrong,” writes Collins.

Collins says she hoped Trump would change once the general election campaign started.

“But 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,” she writes.