Sanders, 78, an impassioned progressive who represents neighbouring Vermont in the US Senate, has long led in opinion polls for the New Hampshire contest. But Buttigieg, a 38-year-old moderate and military veteran who served two terms as mayor of South Bend, Indiana, has gained ground since the Iowa contest.

Deeply trailing in New Hampshire polls, former vice-president Joe Biden kicked off a rainy cold day by visiting with unionised school bus drivers at a depot in Nashua. One driver asked for a picture, saying: “My grandmother is never going to believe me.”

Biden then had the driver call her grandmother, and he spoke to her. The candidate reminisced about driving school buses to earn extra money while a law student at Syracuse University in the late 1960s.

He also won a late endorsement from New Hampshire state Senate President Donna Soucy, who said on Twitter: “Joe has the experience, the tenacity and the empathy that we need in the White House.”

A similar wave of late endorsements before Iowa’s vote didn’t save Biden from a third-place finish.

The Iowa results are still not entirely settled with just a day until New Hampshire votes. Jeff Weaver, senior adviser to Sanders, said late on Sunday that the campaign would make a formal request to the Iowa Democratic Party on Monday to recanvass the results of between 30 and 40 caucus precincts.

A pair of polls released late on Sunday and early Monday showed Klobuchar of Minnesota pulling into third place behind Sanders and Buttigieg following the party’s debate in the New England state on Friday.

“After that debate, something switched,” Klobuchar told MSNBC in an interview on Monday. “It just allowed the people of New Hampshire to see me in a different way.”

Klobuchar has questioned whether Sanders – a self-described democratic socialist – can attract the centrist voters Democrats need to beat Trump, and questioned whether Buttigieg has enough experience as a former mayor to counter the president.

“I’ve won statewide,” including rural and suburban areas, she said, making an appeal for the large number of undecided and independent voters in the state and noting that Trump won narrow victories in several key states in 2016.

A Boston Globe’s poll, conducted with Suffolk University and WBZ-TV, showed Sanders with 27 per cent, Buttigieg with 19 per cent and Klobuchar with 14 per cent among 500 likely voters polled over the weekend. The survey had a margin of error of plus or minus 4 percentage points.

A separate poll by WHDH/Emerson College also showed Klobuchar pulling into third.

The Sanders campaign expects to draw a large crowd in Durham on Monday evening when progressive star US Representative Alexandria Ocasio-Cortez and rock band The Strokes join him for a rally.

US Senator Elizabeth Warren, Klobuchar, Buttigieg, entrepreneur Andrew Yang and the rest of the Democratic field also plan events in the state.

Warren sought to excite voters on Sunday with her detailed “a plan for that” playbook.

“This is not the moment for small ideas, this is not the moment to nibble around the edges of big problems, this is the moment to meet big problems head on with bigger solutions,” she told a crowd in Lebanon on Sunday night. “This is the moment for big, structural change!”