“I am suspending my campaign to become the Democratic Party’s nominee for president,” Mr. Bullock said in a statement. “While there were many obstacles we could not have anticipated when entering this race, it has become clear that in this moment, I won’t be able to break through to the top tier of this still-crowded field of candidates.”

Mr. Bullock, 53, was unable to overcome a late and slow start that hobbled his campaign. Democratic voters never bought into his argument that winning election in a state carried by Mr. Trump was sufficient evidence that he should be the party’s presidential nominee.

An aide said he would not challenge Senator Steve Daines, a Montana Republican facing re-election in 2020, as has been the hope of many Democrats in Washington.