The Democratic presidential race is entering a crucial three-month stretch of the race that will test the mettle of the top candidates and determine whether their lesser-known and lesser-funded rivals will emerge as serious contenders or get kicked to the curb.

The post-Labor Day calendar is filled with milestones, including nationally televised debates, a fundraising deadline, and state- and local-party functions that could prove to be the last chance for some long-shot candidates to show they belong in the race.

“A lot of the campaigns are going to have to do their groundwork in the next 3½ or 4 months in Iowa and New Hampshire,” said Bryce Smith, chair of the Dallas County Iowa Democrats. “This is the prime weather, the sun is still up until 8 o’clock at night and there are still things to do — festivals and farmers markets and stuff. After Thanksgiving, things start to shut down in a way and the sun sets sooner.”

The sun has already set on a few campaigns and odds are that the path to the nomination will be strewn with more casualties by Thanksgiving.

Former Vice President Joseph R. Biden remains the consensus front-runner, having sat atop most national and early primary state polls since entering the race in late April.

The 76-year-old, though, has struggled to shake the idea that his support is fragile and could crumble after a pair of lackluster debate performances and a series of verbal miscues that have reinforced his reputation as the king of gaffes.

“The level of scrutiny of Biden is not going to change,” said Dante J. Scala, a professor of political science at the University of New Hampshire. “That said, Biden is the only one in the race right now who actually has a working coalition that could deliver him the nomination.

“By that I mean African American voters, plus white moderate or blue-collar Democrats,” Mr. Scala said. “So for all the gaffes and stumbles and so forth, he is still in a place where the others need to get to.”

Mr. Biden’s closest rivals, according to polls and fundraising reports, are Sens. Bernard Sanders of Vermont, Elizabeth Warren of Massachusetts, and Kamala D. Harris of California, and South Bend, Indiana, Mayor Pete Buttigieg.

That foursome will have the coming months to prove they can expand their bases of support and build winning coalitions in the early primary states.

“For Warren it is, ‘OK, white progressives like me, they really like me. But that is probably not enough to get me the nomination. So how do I build beyond that?’” Mr. Scala said.

Mr. Biden, Mr. Sanders, Ms. Warren, Ms. Harris and Mr. Buttigieg will headline the Sept. 12 debate in Houston.

The one-night event also will feature entrepreneur Andrew Yang, Sen. Cory A. Booker of New Jersey, former Rep. Beto O’Rourke of Texas, former Housing and Urban Development Secretary Julian Castro and Sen. Amy Klobuchar of Minnesota.

The clock is ticking on these five candidates to make their mark on voters and donors, which will ultimately help determine whether they can air TV and radio ads and seriously compete in each of the early primary states.

Meanwhile, those who failed to make the debate — including Montana Gov. Steve Bullock, Rep. Tulsi Gabbard of Hawaii and New York Mayor Bill de Blasio — are now long shots.

“If you are not on that debate stage, you quickly start to become a memory in the eyes and ears of voters,” said Neil Levesque, executive director of the New Hampshire Institute of Politics and Chief of Staff at St. Anselm College. “That is a difficult position, and that is by design. The DNC wants to have the focus on those who could be a contender, not who is looking to sell a bestselling book.”

The candidates who missed the cut for the debate but are staying in the race will have an opportunity make an impression this week at the New Hampshire Democratic Party State Convention and, after the debate, at the Polk County Steak Fry in Iowa on Sept. 21.

“The steak fry offers a rare opportunity for thousands of people to see and meet the 2020 candidates at a much more affordable price than many of the events,” said Sean Bagniewski, chair of the Polk County Democrats. “We’ve already sold 7,500 tickets to 35 states.”

“For the candidates, it’s a unique opportunity to speak to a national audience and to sway one of the largest crowds of Iowa caucus voters,” he said.

Political insiders, meanwhile, have been left to ponder who will be able to avoid the fate of Gov. Jay Inslee of Washington, Rep. Seth Moulton of Massachusetts and Sen. Kirsten Gillibrand of New York, who dropped out of the race once it became clear they failed to qualify for the third debate.

Mr. Smith said in Iowa that the winnowing “is going to release some tension among voters and caucusgoers who say, ‘OK, that is one person we can check off our list; we don’t have to worry about them anymore.’

“It is nice to clear the field a tad,” he said.

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