Right now, de Blasio has more than 40 percent of the vote, which is the magic number for avoiding a runoff.
But the Board of Elections is collecting voting machines from across the city to check the count. There are also 16,000 absentee ballots that will be added to the tally, hopefully by Monday.
If de Blasio dips below 40 percent, he would face a new fight with second-place finisher Bill Thompson, who is rejecting calls for him to concede.
De Blasio was slated to receive the support Thursday of unions that had previously backed City Council Speaker Christine Quinn, who finished a distant third.
Late Wednesday night, the building service workers' union, SEIU Local 32 BJ, which had previously backed Quinn, endorsed de Blasio.
Republican nominee Joe Lhota gave back-to-back radio and television interviews Thursday morning in which he discussed the "stark" differences between his policies and de Blasio's.
"Bill has a completely different philosophy than I do on how to deal with public schools, how to deal with public safety, how to create jobs, how to deal with taxes. We're as divergence as you can possibly be," Lhota said on the WOR Radio's John Gambling Show.
He told WOR and Fox-TV's "Good Day New York" that his biggest strength is that he'll talk and listen to anybody. He said he wants to restart town hall meeting throughout the city and to get parents involved in the education process.
"This campaign is going to be about change," he told WOR. "Anyone who wants to buy into the conventional wisdom that the other side represents change and I represent status quo will be sadly mistaken. It's absolutely not true."
Lhota, whose party is outnumbered by Democrats 6-to-1 in the city, is trying to project an aura of stability to independents, moderates and business leaders wary of de Blasio's fiery rhetoric. He is a one-time deputy mayor to Rudolph Giuliani and former head of the Metropolitan Transit Authority.
Asked if he will seek the support of Rev. Al Sharpton, Lhota said there was nobody he wouldn't talk to. As for getting Bloomberg's support, he said "anyone who wants to endorse me, that's great." But he said he was running on his own record and did not think endorsements were overly helpful.
(The Associated Press contributed to this report)