And Mr. McGuire and Mr. Donovan, despite being aided by outside spending, have demonstrated little traction in polls. The debate is one last chance to stand out.
The Marist poll found Mr. Adams at 24 percent among likely Democratic voters, when including voters who leaned toward him. Ms. Garcia followed at 17 percent; Ms. Wiley was at 15 percent and Mr. Yang, who had consistently led sparse polling for much of the race, landed 13 percent.
Voters may rank up to five candidates in order of preference, and when several rounds of ranked-choice voting played out in the poll, Mr. Adams came out ahead of Ms. Garcia. But other surveys found different results, and polling is never predictive, much less in a relatively untested ranked-choice scenario.
Gale Brewer, the Manhattan borough president, said voters are still navigating how to order their ballots. She described New Yorkers sitting in Central Park for long conversations with neighbors, trying to reach conclusions.
“They know who their first is, and then they have no clue after that,” she said. “A debate can help with that.”
In theory, ranked-choice voting was supposed to mitigate attacks, because candidates are hoping to have broad appeal and would not want to alienate voters by trashing their first-choice candidates.
Yet the race has seen its share of negative campaigning. Lee M. Miringoff, the director of the Marist College Institute for Public Opinion, reached for an old saying — “the enemy of my enemy is my friend” — to explain how candidates make determinations about which candidates to attack and where to make overtures.
“They have to sort of demonstrate, ‘If you’re with me, stay with me,’” he said. “‘If you’re not with me, here’s why you should join my campaign. If you’re not going to do that, at least put me into the running so I can get into a later round.’”
Emma G. Fitzsimmons and Michael Gold contributed reporting.