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 take on 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.