Correction: An earlier version of this story incorrectly characterized the project at issue in Vincent Orange’s lawsuit against Linda Cropp. It was the city’s baseball stadium project. This version has been corrected.

Democrat Vincent B. Orange defeated Republican Patrick Mara in a special election Tuesday for the at-large D.C. Council seat being held by Sekou Biddle, who trailed in third place.

According to unofficial returns from the city’s 143 precincts, Orange led eight other candidates with about 28 percent of the vote in a race to fill the seat that Kwame R. Brown (D) gave up when he was elected council chairman last year.

Mara received about 26 percent of the vote, while Biddle — whom the D.C. Democratic Committee appointed in January to temporarily fill the seat — garnered about 20 percent.

Democrat Bryan Weaver, 40, a former Ward 1 advisory neighborhood commissioner, received about 13 percent, while Josh Lopez, 27, also a Democrat, finished fifth, with 7.1 percent. The four other candidates — Tom Brown and Dorothy Douglas, both Democrats; Alan Page of the D.C. Statehood Green Party; and independent Arkan Haile — together received less than 5 percent.

If Orange’s margin holds after final absentee and provisional ballots are counted in 10 days, his victory would be a setback for Brown and Mayor Vincent C. Gray (D), who both endorsed Biddle, 39, but were unable to extensively campaign for him this spring as they tried to contain ethical controversies at City Hall.

In an interview after declaring victory before 11 p.m., Orange said his win was a rejection of the city’s leadership. “Georgetown, downtown, Ward 7, Ward 8. They said: ‘You need to back off. We want Orange.’ ”

Orange led even though Mara, 36, who represented Republicans’ best chance in years to win a seat on the 13-member council, carried many majority-white precincts in Capitol Hill and Upper Northwest, where turnout appeared higher than in many other areas.

Mara, who conceded defeat about 11 p.m. Tuesday, thanked his supporters, saying, “We just came up a little bit short.

“Honestly, there were a lot of good candidates in the race, and take away one of these candidates, we probably could have pulled it off,” Mara, a Ward 1 school board member, told The Washington Post. “At the end of the day, it appears Biddle took votes from me.”

Biddle, who noted that he was in the race before Mara, said: “I’m disappointed by the outcome. I’m certainly proud of the effort I, my campaign team and volunteers put in running an energetic first [council] campaign.”

Orange, 54, had little opposition in majority-black neighborhoods in Northeast and Southeast, where his support proved too much for Mara or Biddle to overcome.

He performed surprisingly well in Ward 4, where many upper-income African Americans live and which is home territory to Biddle.

Orange, an accountant and lawyer who lost to Brown in the race for council chairman last year, campaigned as an experienced lawmaker who would help the council balance the budget while being a check on some of the chairman’s influence.

But Orange, who represented Ward 5 on the council from 1999 to 2007, will be returning to a body on which he appears to have few friends. In addition to Gray and Brown, six other council members endorsed Biddle. Several council members expressed reservations about Orange returning.

When last on the council, Orange positioned himself as a bit of a rebel, unsuccessfully suing then-Chairman Linda W. Cropp when she decided that his council committee did not have authority over the construction of the city’s baseball stadium.

On Tuesday, several residents said they voted for Orange because they thought he was experienced and they didn’t know enough about the other candidates.

“Rest of these guys, it’s their first time out,” said George Poynter, 87, who voted at Patterson Elementary School in Washington Highlands, in Ward 8. “We’d be right back where we started.”

Yet Orange struggled to win over voters in neighborhoods in the western part of the city, resulting in an electoral split similar to last year’s mayoral race, in which Gray unseated Adrian M. Fenty (D).

In Chevy Chase, in Ward 4, Leila Gordon said she voted for Biddle in part because she feared Orange’s return to the council.

“I think Mr. Orange is part of the problem with D.C.,” she said after voting at the Chevy Chase Community Center. “I am not confident he has the best interests of the city at heart.”

As expected during a special election, turnout was light despite the 80-degree temperatures and mostly sunny weather. But turnout — about 9.5 percent — exceeded that of an at-large council special election in 1997, when 7.5 percent of the electorate voted. More than 43,000 cast ballots in this election, while only about 25,000 voted in the 1997 election.

The low turnout resulted in an unpredictable contest that gave new voters or those affiliated with a minor party an opportunity to make a strong showing in a heavily Democratic city.

Mara, who unsuccessfully ran for the council in 2008, appeared to have the best chance at beating a field that included six Democrats, an independent and a member of the D.C. Statehood Green Party.

Matt Stone, 39, said that Democrats have too much influence and that he was concerned about how the city is being run. “He seems like he would be a good check on the Democratic council,” Stone said of Mara.

But Mara was competing for many of the same voters that Biddle, a former Ward 4 school board member, and Democrats Weaver and Lopez competed for in Northwest.

In Chevy Chase — a neighborhood crucial to Mara and Biddle — some voters said they were displeased with the direction of city government but divided over whether they wanted a Republican on the council.

Jill Watson, 63, said she considered voting for Mara but decided to support Biddle. “It was between Biddle and Mara, but I will do almost anything to keep from voting Republican,” she said.

City residents also voted Tuesday for representatives to the school board. In Ward 4, D. Kamili Anderson had a 258-vote lead over Andrew Moss, and with all precincts reporting in Ward 8, Trayon White Sr. had a 169-vote lead over Philip Pannell.