A Democrat, John Ewing Jr., appeared on track to win Omaha’s mayoral race on Tuesday, unofficial county results showed, a victory that would end a long period of Republican leadership in a politically divided city that has outsize importance in presidential elections.

Mr. Ewing, the longtime Douglas County treasurer, was leading Mayor Jean Stothert, a Republican who had been seeking a fourth term leading Omaha, which has nearly half a million residents and is Nebraska’s largest city. Though the race was officially nonpartisan, the political affiliations of both candidates were well known to voters.

Ms. Stothert conceded to Mr. Ewing in a speech on Tuesday evening, saying that “we leave a strong foundation for the city that we all love.”

Mr. Ewing was leading by about 10,900 votes on Tuesday night with roughly 87,000 ballots tabulated. Brian W. Kruse, the county election commissioner, said in an email that approximately 7,000 early ballots that were turned in on Tuesday would be counted later, plus perhaps a few hundred provisional ballots.

Every four years, Omaha picks a mayor just after a presidential term begins, bringing more national attention to the race than is typical for a city of its size. With relatively few off-year elections scheduled, the race provided an opportunity for strategists from both parties to gauge voters’ moods — even if much of the campaign focused on municipal nuts-and-bolts issues like street paving, crime and a planned streetcar.

State and national Democrats wasted no time in celebrating Mr. Ewing’s apparent victory, which they described as a sign of energy for their party ahead of next year’s midterm elections.