The Democratic pollster Celinda Lake said that debates can help drive future coverage, resulting in a snowball effect. But she added that candidates’ fortunes will largely be determined on the ground in early-voting states.

“I think that honestly, the debates are having more impact on news coverage than they are having impact on voters,” Ms. Lake said. “Candidates don’t get known by unveiling a good plan at the debate. They become known by immersing themselves in these early states.”

To qualify for November’s debate, candidates will need to reach 5 percent support in two polls in early nominating states, or 3 percent in four early-state or national polls. These are higher thresholds than for any debate thus far. A bump after next week’s debate could, at the very least, help candidates such as Senator Amy Klobuchar of Minnesota or the former housing secretary Julián Castro — who are now foundering in the low single digits — secure a spot onstage next month.

But historically, early debates have been more likely to disqualify a candidate than to provide a lasting jolt of momentum. In 2011, Gov. Rick Perry of Texas pledged in an early debate to eliminate three agencies from the executive branch if elected president but couldn’t remember the name of one of the agencies. He never recovered from the flub. (When Mr. Trump became president, however, Mr. Perry was chosen to head the Department of Energy, the very agency he had forgotten to name.)

Last election cycle, the businesswoman Carly Fiorina enjoyed a strong showing in the second Republican primary debate, seeing a sizable uptick in support in the days afterward. But she could not convert it into a consistent climb in the polls.

“Successful first debate performances create buzz for a candidate but also generate expectations for continued success,” the historian and presidential scholar Bruce Buchanan wrote in an email. “If he or she does O.K. but less well in the second debate, the story is about underperformance.”