They are lawyers and school board members, labor activists and faith leaders, lifelong Democrats and party newcomers. Some of them just turned 18, others are pushing 80.

These are the people who make up the 3,939 delegates to the Democratic National Convention. Some are elected, and some are selected — each state party has its own rules — for what is normally a ceremonial task: nominating their candidate for president.

But in the — still unlikely — scenario in which President Biden steps down as the nominee, they will suddenly be charged with picking a new nominee.

Most of these delegates did not set out to become kingmakers in the Democratic Party, but rather to be part of a pro-Biden slate pledging support to the president. But in the event Mr. Biden drops out, they would be vaulted from the obscurity of extras at a quadrennial television extravaganza into a group with the fate of the party — and, in the view of many Democrats, the future of the country — on their shoulders.

Many are loath to even consider that as an option, remaining steadfastly loyal to the president as he affirms his commitment to remaining in the race. And some find themselves overwhelmed by the possibility.

Phil Swanhorst, the chair of the Eau Claire County Democrats in Wisconsin and a first-time delegate, said that “with all the turmoil going on,” he did not want to discuss what he would do as a free delegate if released from his pledged status. Instead, he said he would follow the guidance of Ben Wikler, the chair of the state party.

Ronald Martin, a social studies teacher and member of the National Education Association, a teachers’ union, said he was wholeheartedly behind the president, dismissing Mr. Biden’s debate performance as simply a “bad night” — echoing the president’s words — and not representative of anything else. But forced to make a decision in the event that Mr. Biden withdraws, Mr. Martin said he would take a step back and assess the entire field rather than immediately vote for an alternative.

“I would respect President Biden’s decision, whatever he says, but again, I’d sit and listen to everything,” Mr. Martin said, adding that defeating former President Donald J. Trump remained the goal.

In a speech in Wisconsin on Friday, and in an ABC News interview that aired later in the day, Mr. Biden made it clear he had no intention of withdrawing. “I’m staying in this race,” Mr. Biden told the Wisconsin crowd, to cheers. “I’m not letting one 90-minute debate wipe out three and a half years of work.”

Almost all of the delegates — roughly 99 percent — are pledged to Mr. Biden, reflecting the popular vote in their state primaries. They are not free to support another candidate of their choosing, unless Mr. Biden withdraws. While there is a so-called conscience clause in the rules, permitting delegates to break with their delegations, it is rarely, if ever, exercised.

“This is not 2016, and it’s not 2008, when you had a split delegation,” said Donna Brazile, a former chair of the Democratic National Committee. “This is a Biden convention.”

There are also roughly 700 “automatic delegates” — formerly and commonly known as superdelegates — who are chosen because of their roles in the party. Governors, senators, members of Congress, state party chairs and other high-ranking members within the party make up this contingent, though they do not vote in the first round of nominations.

The final list of the delegates has not been released by the Democratic National Party, but a review of delegate lists released by state parties shows a diverse slate, as required by party rules.

Take the Wisconsin delegation: Among the 95 delegates, there are more than 20 local government officials, 11 current and former educators, nine labor leaders, six college students and a former executive of the Milwaukee Bucks.

Trevor Jung, one of those local government officials — he is the transit director for Racine, Wis. — said he had been involved in politics since he was 12, when his single father used to drop him off at the local Democratic headquarters. Having been a first-time delegate in 2020, Mr. Jung cannot fathom a ticket without Mr. Biden at the top and is unsure what he would do in the event of an open convention.

“I have not given it much thought, and it’s in part because I think President Biden will be our nominee,” he said. “President Biden had a bad night, and Donald Trump had a bad presidency.”

Amaad Rivera-Wagner, the chief of staff to the mayor of Green Bay, Wis., and a second-time Biden delegate, recalled receiving death threats in 2020 because people believed he had rigged the election.

“Yes, there is turmoil, but the convention feels clear to me,” Mr. Rivera-Wagner said, adding that Mr. Biden would have the best chance against Mr. Trump. While he does not believe Mr. Biden will step down, he said he would “explore my conscience but follow Democrats’ suggestion” in that unlikely event.

Most state delegations have many representatives who have served at past conventions. Judy Mount, who said she was the “first African American in the state of Florida — since Ponce de Leon got here — to be first vice chair” of the state Democratic Party, has served at every convention since Barack Obama’s first convention in 2008.

She said she remained a steadfast supporter of Mr. Biden and would follow his guidance if he were to drop out.

“Only if he makes that announcement,” Ms. Mount, 64, said. “Because I have the utmost respect for that young man.”

L. Jeannette Mobley, a delegate from Washington, D.C., who said she had also been a delegate for Mr. Obama in 2008, was similarly loyal to Mr. Biden, saying he had done more “in his first three years than most presidents” and brushing off his debate performance.

Were Mr. Biden to drop out, she said, she would also follow his recommendation.

“If he makes the decision to withdraw, I’m sure he’s going to probably come out with a recommendation,” Ms. Mobley said, adding of Vice President Kamala Harris: “Probably Kamala would be the best person to run. She’s very capable. Don’t get me wrong about that. Although I really have some concerns about whether or not America is ready for a woman president.”

Ms. Mobley mused that if Ms. Harris were to pick “one of the other individuals, like Newsom or Shapiro, then we still have a winning ticket,” referring to Gavin Newson, the governor of California, and Josh Shapiro, the governor of Pennsylvania.

Dave Jacobson, a delegate from Florida who is one year younger than Mr. Biden, called last Thursday’s debate “devastating” but was heartened by the president’s more energetic rally the next day and remains steadfastly in support. Like Ms. Mobley, if forced to vote for someone else by Mr. Biden’s withdrawal, Mr. Jacobson said that “the vice president would be the logical choice.”

“It would be a travesty if something were to happen that Joe bowed out and that Kamala would not be our nominee,” Mr. Jacobson said. “If she is not, the Democratic Party will face a pretty devastating election on Nov. 5.”

June Kim, Eli Murray, Andrew Park, Helmuth Rosales, Elena Shao and Amy Schoenfeld Walker contributed reporting. Alain Delaquérière contributed research.