Badly outspent on the airwaves here, Mr. Biden has started taking direct aim at both Mr. Buttigieg and Mr. Sanders in a revamped stump speech, a first for a candidate who has led the field in most national polls and largely focused his fire on President Trump. Ms. Warren, however, is amplifying her message about being the Democrat who can unify a badly fractured party but is so far avoiding direct criticism of her rivals.

Mr. Biden risks coming off as overly negative, always a gamble in a multicandidate race where third parties can benefit from such attacks. But there are dangers for Ms. Warren as well, if she appears to be acting without enough urgency, given Mr. Sanders’s lead in New Hampshire polls and the expectations she faces coming from next-door Massachusetts…

“Donald Trump is desperate to pin the socialist label of socialist, socialist, socialist on our party,” Mr. Biden said, alluding to Mr. Sanders, a self-avowed democratic socialist. He argued that every Democrat on the ballot “will have to carry the label” in November…

“I have great respect for Mayor Pete and his service to this nation,” Mr. Biden said. “But I do believe it’s a risk, to be just straight up with you, for this party to nominate someone who’s never held an office higher than mayor of a town of 100,000 people in Indiana.”