WASHINGTON — The unofficial end of summer kicks off a more intense phase of a Democratic presidential race that has been led by moderate Joe Biden, with progressives Bernie Sanders and Elizabeth Warren in a battle for second as the rest of the sprawling field struggles to break through.

“There’s a real sense that time is running out after Labor Day,” said Alex Conant, a public relations consultant who was a top aide on the 2016 presidential campaign of Sen. Marco Rubio, a Florida Republican. “Voters become more engaged. Before Labor Day most voters are content to watch the show. After Labor Day they start picking a team.”

He added, “Especially if you’re a second- or third-tier candidate who’s been waiting to make a move, the time is now.”

Democrats have essentially culled the field to a four-person race with a few wild cards showing signs of life and others hanging by a thread. Party members are eager to nominate a candidate they believe can beat President Trump, keeping Biden atop polls even amid concerns about his age and propensity for gaffes.

Sanders and Warren are behind, splitting Democrats hungry for swift and far-reaching progressive change, while Kamala Harris has settled into a distant fourth place.

No one else is polling above 10% in a field that has in recent weeks dwindled from 24 to 19 candidates.

At the same time, the economy remains strong but shows signs of sputtering that make Trump allies nervous.

The upcoming debate on Sept. 12 in Houston will be the first time all the front-runners are on the stage together. Many are anticipating a confrontation between Biden and Warren, who’ve never shared the podium but have a contentious history and represent a stark choice between the moderate and liberal wings of the party.

So far, Biden’s resilient lead has defied critics who suggested that the third-time presidential candidate was on track for another flameout.

Voters “kind of know he’s the guy who can win important swing voters,” said John Anzalone, Biden’s pollster and adviser. “They feel like there’s too much risk in some of the candidates who might be considered more left.”

In a recent Quinnipiac University poll, 54% of registered voters said they’d vote for Biden, while 38% said they would vote for Trump. But Sanders would also win the popular vote, 53% to 39%; as would Warren at 52% to 40%, Harris at 51% to 40%, and Buttigieg at 49% to 40%.

“One of the overwhelming things I hear about Joe Biden is that he definitely acts and talks his age — which isn’t necessarily a bad thing for some voters, but for other voters, especially for younger voters, is not appealing,” said Bryce Smith, the chairman of the Dallas County Democratic Party in Iowa, just outside Des Moines.

“He might have some really good ideas,” he said, “but the way he talks about them may not be as motivating or energizing as Elizabeth Warren or Bernie Sanders who are similar in age.”