WASHINGTON — U.S. Sen. Susan Collins said Monday she will not vote for fellow Republican Donald Trump for president.

“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,” Collins said in an opinion piece in The Washington Post.

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

Before the conventions were held, Maine’s Republican senator said she wasn’t ruling out the possibility of voting for Democrat Hillary Clinton, though pointing out that it was “extremely unlikely.”

In her opinion piece, Collins said she did not support either major-party candidate.

The New York Times reported Monday evening that Collins’ opposition could make it more difficult for Trump to compete in Maine, where he campaigned last week in an effort to capture at least one Electoral College vote.

Several of her Senate colleagues — including Nebraska Sen. Ben Sasse, South Carolina Sen. Lindsey Graham and Illinois Sen. Mark Kirk — also have ruled out voting for Trump. Arizona Sen. Jeff Flake has repeatedly said he cannot back Trump yet, reports CQ-Roll Call.

Former U.S. Secretary of Defense and Maine U.S. Sen. William S. Cohen also has criticized Trump’s recent performance, Maine Public Broadcasting Network reported last week.

Cohen, who served as secretary of defense under President Bill Clinton from 1997 to 2001, is a Republican who says he continues to support Republican candidates. But Cohen said it is unlikely that he’ll be voting for Trump. Cohen himself represented Maine in Congress from 1973 to 1997, both in the U.S. House of Representatives and as a member of the Senate.

Cohen said he finds it “very difficult” to have Trump at the top of the ticket. “I have found very little, if anything, that I could find supporting him as a commander-in-chief. … That’s not the leader I want to see for this country,” MPBN reported.

Also on Monday, a group of 50 former national security officials, all of whom have served Republican presidents from Richard M. Nixon to George W. Bush, signed an open letter calling Trump unqualified to be president and warning that, if elected, “he would be the most reckless President in American history.”

The letter offers a withering critique of the GOP nominee, saying he “lacks the character, values and experience” to be president. The signatories declare their conviction that he would be dangerous “and would put at risk our country’s national security and well-being.”

They state flatly that none of them intend to vote for Trump in November. Some have decided to vote for Hillary Clinton, while others intend to sit out the election or write in another name, said John Bellinger III, a former legal adviser to Condoleezza Rice and the writer of the letter’s first draft.

“We also know that many have doubts about Hillary Clinton, as do many of us,” the letter says. “But Donald Trump is not the answer to America’s daunting challenges and to this crucial election. We are convinced that in the Oval Office, he would be the most reckless President in American history.”

In a statement, Trump said the letter writers share the blame for “making the world such a dangerous place.”

“They are nothing more than the failed Washington elite looking to hold onto their power, and it’s time they are held accountable for their actions,” he said.

Trump said the former officials — along with Clinton — took part in the decisions that led to the invasion of Iraq, the deaths of Americans in Benghazi, Libya, and the rise of the Islamic State.

“Yet despite these failures, they think they are entitled to use their favor trading to land taxpayer-funded government contracts and speaking fees,” he said.

Although no former secretaries of state signed the letter, it carries the signatures of Michael Chertoff and Tom Ridge, former secretaries of homeland security; Michael Hayden, a former director of the CIA and the National Security Agency; John Negroponte, a former director of national intelligence and deputy secretary of state; Robert Zoellick, who also was a deputy secretary of state and president of the World Bank and the U.S. trade representative under George W. Bush; Carla Hills, the U.S. trade representative under George H.W. Bush; and William H. Taft IV, a former deputy secretary of defense and ambassador to NATO under the elder Bush.

Also signing the letter were several aides who were senior advisers in the White House, State Department and Pentagon. Among them were Eric Edelman, a national security adviser to then-Vice President Richard B. Cheney, and Bellinger, who worked closely with Rice when she was secretary of state and when she was on the NSC.

Bellinger said that some involved with the letter wanted to wait until September to release their views but that the candidate’s behavior in recent weeks — from his comments on NATO to inviting Russian intelligence to hack Clinton’s emails — galvanized them to move sooner.

“This is not about NATO, it’s not about trade, it’s not about Russia, it’s not about cyber. We really wanted to focus on the character, temperament and judgment that we have seen are required of good presidents,” Bellinger said.

Although the signatories all served Republican presidents, many of the criticisms echo those being leveled by the Clinton campaign.

“He appears to lack basic knowledge about and belief in the U.S. Constitution, U.S. laws and U.S. institutions, including religious tolerance, freedom of the press and an independent judiciary,” the letter says of Trump.

Later on, it adds, “At the same time, he persistently compliments our adversaries and threatens our allies and friends. Unlike previous Presidents who had limited experience in foreign affairs, Mr. Trump has shown no interest in educating himself. He continues to display an alarming ignorance of basic facts of contemporary international politics.”

The letter said Trump “lacks the temperament to be President,” and gave a scathing assessment of his ability to take advice, discipline himself, control his emotions and reflect before acting.

“He is unable or unwilling to separate truth from falsehood,” the letter states in a particularly pointed criticism of Trump’s personal traits. “He does not encourage conflicting views. He lacks self-control and acts impetuously. He cannot tolerate personal criticism. He has alarmed our closest allies with his erratic behavior. All of these are dangerous qualities in an individual who aspires to be President and Commander-in-Chief, with command of the U.S. nuclear arsenal.”

Washington Post writer Carol Morello contributed to this report.