MANCHESTER, N.H. — Joseph R. Biden Jr.’s backers roared supportive slogans and banged on drums as they camped outside Southern New Hampshire University Arena. Backers of Senator Elizabeth Warren formed a whole band. A fan of Senator Amy Klobuchar admonished passers-by to consider electability, and a banner associated with Senator Bernie Sanders that highlighted his own standing in the polls appeared aimed at drawing a contrast with Mr. Biden.
The New Hampshire Democratic Party State Convention was expected to draw 19 of the presidential candidates and some of the state’s most committed party activists — including more than 1,200 delegates — to its gathering here Saturday, offering an early test of campaign organization and enthusiasm in a contest that is traditionally a must-win for candidates from neighboring states. This cycle, that includes Mr. Sanders of Vermont, who won New Hampshire by a wide margin in 2016, and Ms. Warren of Massachusetts, whose ground game is often regarded as the most extensive in a contest that many party officials describe as still fluid.
Both of those New Englanders were slated to speak later on Saturday. Mr. Biden, the former vice president, was the first of the presidential contenders to speak, and he received a polite if not raucous reception as attendees trickled into the arena, which was not yet full on Saturday morning.
Taking aim at President Trump, he said: “We cannot and I will not let this man be re-elected.” The audience cheered, boosted in part by a contingent of representatives from a firefighters’ union, the International Association of Fire Fighters.
Mr. Biden has led in most polls here since entering the race — though the surveys have been relatively few. He is seeking to build a coalition that would include blue-collar voters, moderates and other Democrats who believe his more centrist brand offers the most promising path to defeating Mr. Trump, in contrast to the more progressive coalitions Ms. Warren and Mr. Sanders are seeking to engage. Yet his own campaign last week sought to downplay expectations in New Hampshire, noting the “home-field advantage” that Democrats from neighboring states have historically enjoyed.
“There’s that sense of, we know who Joe is and we trust him,” said former State Senator Sylvia Larsen, the former New Hampshire Senate president. “There’s still a little bit of people still looking around to say, ‘Well, O.K., so what else is out there? Where are the voices? Who else might be a voice?’”
New Hampshire voters are discerning, relishing their responsibilities as voters in the nation’s first primary contest, following the Iowa caucuses. Given the huge cast of candidates, many voters are in no mood to commit this early.
“I think of it like getting an ice cream sundae and trying to decide what toppings you want to put on your ice cream, because they’re very good choices, but I just can’t make up my mind,” said Jeanne Brown from Goffstown, N.H., who attended a Warren event in Bow, N.H., over the summer. “You know, I don’t know if I want the gummy bears or the sprinkles.”
That same indecision was on display outside the arena on Saturday morning, as activists and other voters rattled off lists of top three choices, but repeatedly stressed that their rankings could change. For at least one voter, they changed in a matter of moments.
“Pete, Warren and then Biden,” said Tim Ashe of Somersworth, N.H., referencing Mr. Buttigieg, the mayor of South Bend, Ind. “That may change.”
When asked why Mr. Buttigieg was his first choice, he replied, “I was looking at a Pete sign when I said it.”
He went on to say that “if I had to vote today, it would probably be for Warren,” pointing to her “gumption” and his belief that she is “truly looking out for the little guy.”
His wife, Laurie Ashe, said that she had supported Republican John Kasich, the former governor of Ohio, in the primary in 2016 and then voted for Democrat Hillary Clinton in the general election. Now, she said, she is supporting Ms. Warren because “regular people cannot work their way into the middle class anymore.”
Mr. Sanders and Ms. Warren were slated to speak later Saturday, and their appearances were certain to be closely watched.
“Both do have strong organizations here in New Hampshire,” said Kathy Sullivan, the former New Hampshire Democratic Party chairwoman, in an interview ahead of the gathering. “I’m just curious to see, beyond Warren and Sanders, who else has a lot of people there.”
Ms. Sullivan said she was interested to see if anyone would give “a rip-roaring” speech that would get “people up on their feet cheering.”
As Mr. Buttigieg took the stage, his vocal supporters did just that.
Jonathan Martin contributed reporting from Bow, N.H.