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A count by electoral authorities from 99.5 percent of constituencies gave a comfortable lead to Civic Platform with just more than 39 percent support. That puts it well ahead of its main rival, the conservative Law and Justice party of former Prime Minister Jaroslaw Kaczynski, with nearly 30 percent support. The twin brother of former President Lech Kaczynski, who died in a plane crash last year, acknowledged his defeat Sunday night.

The count shows that Mr. Tusk’s pro-market Civic Platform and its coalition partner, the Polish People's Party, would hold a narrow majority in the 460-member lower house of parliament if they continue their coalition.

The Polish People's Party — a socially conservative group that represents farmers’ interests — had more than 8 percent support in the near-final count. Jointly, the two parties could have 234 seats in the 460-member lower house, or Sejm.

The two parties enjoyed a drama-free relationship, at least in public, that added to the government’s stable image — and contrasted with the public fighting that had marred past governments. Though the parties had some disagreements, they managed to work them out behind closed doors, according to Warsaw Mayor Hanna Gronkiewicz-Waltz, a prominent Civic Platform member.

The new left-wing party, Palikot's Movement, was in third place with 10 percent. Led by entrepreneur and maverick lawmaker Janusz Palikot, the party has gained popularity by promising to support gay rights, liberalize the country’s strict abortion laws and legalize marijuana.

The only other party that would make it into parliament is the Democratic Left Alliance, which won slightly more than 8 percent of the votes cast.

That figure marks a sharp decline for the party, the successor to the Communist Party, which ruled Poland before 1989. It has held power off and on since communism fell in 1989 but has seen its popularity decline steadily in recent years. In this election it appeared to lose voters to Palikot's Movement, which shares many of its ideological positions.

Party Chairman Grzegorz Napieralski announced Monday that he would not put himself up for re-election when his party next votes on its leadership.

Monika Scislowska contributed to this article.