WASHINGTON — John Deeth is the sort of Iowa activist who keeps up with every new twist in the Democratic presidential primary, but there was no way he was going to devote two evenings to watching the first round of debates last week.
Instead Mr. Deeth, a 55-year-old from Iowa City who has volunteered for every Democratic caucus since 1992, spent one debate night watching “Goodfellas” and the other at the grocery store.
“Until this field is narrowed down to serious people, I’m not going to spend four hours watching them,” Mr. Deeth said. “I’m not going to suffer through Andrew Yang.”
Mr. Deeth’s frustration highlights a central challenge the Democratic Party faces as it tries to identify the strongest candidate to face President Trump: The slate of 24 contenders is too unwieldy for a constructive debate, many activists and strategists say, and too large for most voters to follow. And with a leadership vacuum at the top of the party, there is no one to elevate candidates with an endorsement, or help steer third-tier candidates out of the race when they’ve reached their plausible expiration date.
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Former President Barack Obama, an influential voice among many Democrats, is sitting out the primary. The Clintons, a once-dominant party presence, are largely unwelcome this time around. In Washington, Speaker Nancy Pelosi is focused on keeping the House in order, and Chuck Schumer, the Senate minority leader, has failed to recruit presidential candidates like Gov. Steve Bullock of Montana and former Representative Beto O’Rourke of Texas into potentially winnable Senate races.
The only kingmaker willing to crown candidates so far has been Oprah Winfrey.
The media mogul did a glowing interview with Mr. O’Rourke when the Texan was at the near-peak of his popularity in February. Since then she has moved on to Pete Buttigieg, the mayor of South Bend, Ind., telling the Hollywood Reporter that she instructed the director Steven Spielberg to “look him up.” (She hasn’t yet touted Marianne Williamson, her onetime spiritual adviser.)
Of the party’s living former presidential nominees, just Walter Mondale has weighed in on the race, endorsing a fellow Minnesotan, Senator Amy Klobuchar. The rest are keeping their distance from the messy primary, which post-debate polling shows has bifurcated between a top tier of five candidates and everyone else vying just to qualify for the party’s fall debates.
Only Mr. Obama has the stature within the party to influence the primary scramble, former Gov. Ed Rendell of Pennsylvania said.
“If Barack Obama stood up and said, ‘You other Democrats, stop going after Joe Biden for his record from 30 years ago, I knew all this stuff and I picked him to be vice president of the United States,’ that might do it,” he said. “But I don’t think the president feels it would be right for him.”
Mr. Obama has held private conversations with most of the Democrats running and speaks regularly with former Vice President Joseph R. Biden Jr. He is not likely to engage in a public way until late in the primary process.
The Clintons’ connection to the primary is more complicated. Former President Bill Clinton, his legacy tarnished by sexual indiscretions and allegations of sexual assault (which he denies), is persona non grata on the campaign trail in the #MeToo era.
Hillary Clinton is still popular with many Democrats, including women, but her Electoral College loss to Mr. Trump limits her influence over party politics and on shaping the 2020 Democratic field; she has met or talked with several of the candidates, but they generally do not see her as someone who could call shots in the race.
Former Senator Barbara Boxer of California said the calls to cull the field are a symptom of tension between a right-now society and a nominating process designed to take 18 months or more.
“Everybody wants instant answers,” Ms. Boxer said. “Who is going to be left, who is going to win. We don’t know any of that. If you’re getting squeezed out of the debates, do something fabulous and figure it out to get attention.”
The debate format itself, created by the Democratic National Committee, has added to the sprawling nature of the primary. The relatively low thresholds for the first two debates have provided an incentive for low-polling candidates to stay in at least through the end of July.
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While NBC’s high ratings for the first set of debates suggest high interest in the presidential campaign, party leaders in early voting states like Iowa — which have had the most exposure to the 24 Democrats in the race — say the oversaturation is counterproductive.
“Ten of them should drop out now,” said Claire Celsi, an Iowa state senator. “It’s not really fair to the other people. The debates are just unmanageable.”
After last week’s debates, Ms. Celsi posted to Facebook a list of 10 Democratic candidates who she said should consider dropping out. Of Ms. Williamson, Ms. Celsi wrote, “Love can’t pay the bills.” And she said Senator Michael Bennet of Colorado suffers from “white guy can be president if he wants syndrome.”
Other Democrats rebutted the idea that candidates should be persuaded to end their campaigns.
Deval Patrick, the former Massachusetts governor who considered a 2020 presidential run, said the decision to drop out can only belong to the candidate.
“In a successful democracy, the candidate or the voters, and only they, should determine when it’s appropriate for a candidate to drop out,” Mr. Patrick said. The field, he said, “is not too big.”
Veterans of the bitter 2016 primary between Mrs. Clinton and Senator Bernie Sanders of Vermont recall Mr. Sanders’s complaints that the Democratic machinery was rigged against him, and wince at the possibility that claims of favoritism or disputes over viability could again prove divisive.
For that crowd, the party’s debate thresholds — due to rise to 130,000 unique donors and 2 percent in at least four polls by September, double the requirements for the first two debates — is a recipe for producing grievances from candidates who don’t qualify.
“It is crazy dangerous for the D.N.C. to take on the role of limiting the field,” said Jennifer Palmieri, who was communications director for Mrs. Clinton’s campaign. “Even if you have good intentions, the unintended consequences of that could be pretty devastating.”
Donna Brazile, who became chairwoman of the D.N.C. after revelations that her predecessor, Debbie Wasserman Schultz, had favored Mrs. Clinton, said people should remind themselves the second-tier candidates won’t be around much longer.
“The biggest part is going to be dominated by the top four or the top two,” she said. “It’s like the N.F.L., we don’t complain about the number of teams we see during the season, we want to know who has the best players to get to the playoffs.”
The Democratic Party chairman, Tom Perez, said he considered the debates a roaring success, marked by high TV ratings and a focus on policy issues, while avoiding the attack-driven fireworks that characterized the 2016 Republican debates.
The D.N.C. has been squeezed between those who think there are too many candidates and others who say the more stringent criteria for future debates is too limiting. Mr. Perez has said over and over again that the party’s rules have been set for months and that he is not going to change them.
“You can’t set rules and then make new rules that would violate our commitment to fairness, transparency and consistency,” Mr. Perez said. “We explained all of the rules to the candidates back in February. Everybody nodded, understood and agreed.”
All three 2020 candidates Mr. Schumer failed to convince to run for the Senate are now mired among the race’s also-rans. Former Gov. John Hickenlooper of Colorado purged his top campaign staff this week. Mr. O’Rourke registered less than 1 percent in the latest Suffolk University poll of Iowa Democrats. Mr. Bullock, the Montana governor, didn’t even qualify for the first round of debates.
For any candidate who is considering leaving the race, the comedian Samantha Bee offered a prominent platform: her late-night show on TBS.
She invited candidates to announce they are leaving the race on her show, and promised to throw them a dropout party with balloons, music and a cake of the candidate’s choosing.