A winner might not be called until Wednesday or later.

With the Chicago mayoral race virtually tied, it appeared increasingly possible that it could be Wednesday, or even later, before a winner is projected.

“It’s unlikely that a candidate will concede tonight,” Max Bever, a spokesman for the Chicago Board of Elections, said Tuesday evening.

With 84 percent of the expected votes in, Mr. Johnson had 50.7 percent of the vote; Mr. Vallas had 49.3 percent.

Thousands of mail-in ballots, which were permitted to be postmarked as late as Tuesday, still have to be counted, a process that could take days or weeks. News organizations could project a winner sooner.

Mr. Bever said ballots must be counted by the Board of Elections by April 18; an official proclamation of results will occur around a day or two later.

Given the closeness of the margin, a recount is also possible, but that process can only be initiated by a candidate after the official proclamation of results.