Donald Tusk is set to become the first Polish prime minister since the fall of communism in 1989 to serve two terms in a row.
His Civic Platform appears to have won some 39 percent of votes in parliamentary elections on October 9, according to an exit poll.
The opposition Law and Justice party led by Jaroslaw Kaczynski got some 30 percent, based on 93 percent of the vote counted.
Final official results are not expected until later in the day, but Tusk has already claimed victory and Kaczynski has conceded defeat.
The conservative Civic Platform is expected to renew a coalition with the Peasants Party, which won about 8 percent of the vote.
The coalition was credited with steering Poland smoothly through the 2008-09 global financial crisis without dipping into recession.
"I know that the next four years will be an even bigger challenge because we will need to double our pace," Tusk told his supporters in Warsaw on October 9. "We will need to act with twice the speed, firstly because the Polish people have the right to an even higher quality of life and to an even higher level of life."
Tusk has good ties with German Chancellor Angela Merkel and pursued warmer relations with Russia, despite strains over a plane crash there last year that killed then-President Lech Kaczynski, Jaroslaw Kaczynski's twin.
Analysts say the main election surprise was the 10 percent of the vote won by a new liberal grouping, Palikot's Movement.
Its founder and wealthy businessman Janusz Palikot has criticized the powerful Roman Catholic church and backed causes such as gay rights and the legalization of soft drugs, striking a chord among young voters.
compiled from agency reports