2020 Democrats Fan Out Across Iowa and New Hampshire on Labor Day

“I think to elect anyone else is a foolish venture,” he said.

How Mr. Sanders, Mr. Biden and Ms. Warren compete and contrast themselves with one another is one of the biggest questions of the autumn phase of the campaign. That mini-contest among the leading candidates will also reveal whether there is enough oxygen for any of the 17 other hopefuls to catch fire in the five months before the Iowa caucuses.

Mr. Biden’s summer has hardly been a smooth ride. Mr. Booker and Ms. Harris attacked his record on race. Senator Kirsten Gillibrand did so on gender issues, before dropping out last week. Mr. Buttigieg, who is 37 years old, has built his campaign on an unsubtle age contrast with Mr. Biden, who is 76.

Yet more than four months after he opened his campaign, Mr. Biden remains the leader. Few of the Democratic county leaders and party activists who populate Iowa’s political class say they are thrilled with the former vice president, and many say there’s little enthusiasm for his candidacy, yet he remains atop almost all polls of Democrats, in Iowa and elsewhere.

Still, a belief that Mr. Biden’s campaign will crater permeates the other presidential campaigns. Eventually, officials with rival campaigns say in private conversations, voters will find Mr. Biden an unsteady hand, his gaffes too risky in a contest with President Trump.

None of that has happened yet.

In two weeks, the top three candidates will appear on a debate stage for the first time when the top 10 Democrats gather in Houston. ABC’s moderators are certain to draw contrasts between Mr. Biden and Ms. Warren, who have clashed on economic issues across the last three decades. Mr. Sanders, though ideologically more similar to Ms. Warren, shares a base of lower-educated and lower-information voters in the Democratic contest with Mr. Biden.

Amid this top-three dynamic, all of the Democratic presidential candidates will find themselves facing new competition for voters’ attention.

Gone are the state and county fairs, with hordes of would-be voters awaiting candidate speeches from soap boxes.