After he surged to the top of the pack in recent weeks, de Blasio's campaign staff maintained they expected him to end up in a runoff, even though some polls showed him above the 40 percent threshold. Thompson's campaign insisted their candidate was being underestimated by the pollsters, and although he does appear set to outperform those polls, it was unclear whether that would be sufficient to force a runoff with de Blasio.
All of the leading Democratic candidates were polling far ahead of their Republican rivals in hypothetical matchups of November's general election.
With the race this close, it will come down to the final few percentage points, and perhaps to a count of paper ballots used by absentee voters as well as those who encountered issues at the polls. The tight margins also raise the possibility that the race will head to court. The candidates had lawyers standing by, ready to scrutinize the Board of Election's count.
The other two top Democratic candidates, City Council Speaker Christine Quinn and former Rep. Anthony Weiner (D-NY) both conceded late Tuesday. In another major contest, the comptroller's race, the Associated Press declared Manhattan Borough President Scott Stringer victorious over former Gov. Eliot Spitzer.