“It’s up to you, Massachusetts, to decide what to do,” Ms. Warren instructed.

Supporters looked back at her, murmuring. She realized why. “And to the people of New Hampshire!” she amended.

On the eve of a contest she had hoped to win (and probably will not, according to polls) — one week removed from a caucus she had hoped to win (and certainly did not, according to Iowans) — Ms. Warren has arrived, almost imperceptibly, at a precarious stage.

In a primary adjoining her own state, it is Senator Bernie Sanders, another New Englander, and Pete Buttigieg, the former mayor of South Bend, Ind., who are leading in polls. Hoping to turn New Hampshire into a two-person race, the pair have been slinging fresh insults: Mr. Buttigieg suggested on Monday that nominating Mr. Sanders would “risk alienating Americans at this critical moment.” Mr. Sanders, contrasting his online fund-raising army to Mr. Buttigieg’s cadre of high-dollar donors, said he would not “go to rich people’s homes and get advice from millionaires and billionaires.” And after a chaotic virtual tie in Iowa, both campaigns on Monday requested a recanvass of certain caucus precincts.