Senator Susan Collins, Republican of Maine, announced Monday that she would not vote for Donald J. Trump in the presidential election, dealing another blow to Mr. Trump as he tries to unite his party and win more support from women.

Ms. Collins revealed her decision in an opinion column for The Washington Post published Monday evening, saying that Mr. Trump’s proclivity for bullying and invective made it impossible for her to support him. She said she believed having Mr. Trump as president would make “an already perilous world” even more dangerous.

Ms. Collins is the most senior senator to split publicly with Mr. Trump, and her message of censure could send a message to other Republicans that it is safe to shun the party’s presidential nominee. She is one of the few moderate Republicans remaining in the Senate and one of only two from New England, along with Kelly Ayotte of New Hampshire, whom Mr. Trump, after some hesitancy, endorsed late last week.

Now in her fourth term, Ms. Collins also led the Senate committee that oversees homeland security, adding additional weight to her criticism of Mr. Trump, who has strained to convince middle-of-the-road voters that he is prepared to serve as the country’s commander in chief.