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.”