Representative Jared Golden of Maine, a centrist Democrat, called for a ban on assault weapons Thursday afternoon, reversing a long-held stance after 18 people were killed in a mass shooting in Lewiston.

It was both a remarkable change on a polarizing issue for a politician who has held onto one of the most competitive seats in the House and a familiar response for a person deeply shaken after a mass shooting happened close to home.

Mr. Golden, a Marine Corps veteran, has repeatedly broken with his party to oppose legislation that would ban assault weapons, a policy that Democrats have repeatedly tried and failed to revive in the nearly two decades since it lapsed. Last July, he was one of just five Democrats to oppose such a measure, which has failed to secure enough Republican votes in the Senate.

That position, Mr. Golden said on Thursday, reflected in part “a false confidence that our community was above this, and that we could be in full control, among many other misjudgments.”

“The time has now come for me to take responsibility for this failure,” Mr. Golden said. He asked “for forgiveness and support” from the people of Lewiston and the loved ones of the victims and survivors, as he pledged to work toward reviving the ban.

Standing by him at a news conference, Senator Susan Collins, a centrist Republican, declined to back a ban on assault weapons. The senator, who helped negotiate a compromise measure that broke a decades-long stalemate on any legislation aimed at changing the nation’s gun laws last year, instead said lawmakers should look at banning “very high capacity magazines.”

“There’s always more that can be done,” she said.

Representative Alexandria Ocasio-Cortez, Democrat of New York, one of the most liberal lawmakers in the House, called Mr. Golden’s comments “powerful, brave and moving” on social media.

But it is all but guaranteed that a divided Congress will not move on any gun legislation given deeply entrenched conservative opposition to any measure that could be perceived as an infringement of the Second Amendment.

The 2022 compromise law — which expanded the background check process, put aside millions of federal funds toward mental health and the implementation of so-called red-flag laws, among other changes — fell short of the sprawling changes Democrats have demanded. But only 29 Republicans — 15 in the Senate and 14 in the House, many of them since retired or defeated — voted in favor of that bill.