By Alan Charlish and Anna Wlodarczak-Semczuk
WARSAW (Reuters) -Nationalist opposition party Law and Justice (PiS) came first in Poland's local government elections, an exit poll showed on Sunday, in a setback for Prime Minister Donald Tusk's ambitions to cement his grip on power.
If confirmed, the results would dash Tusk's hopes of leading his Civic Coalition (KO) grouping to first place in the election independently of its partners in the ruling alliance.
While PiS came in ahead of KO, it was still well behind the combined score of the three groupings that make up the coalition government, and appeared set to lose power in some regional governments.
PiS scored 33.7%, according to the Ipsos exit poll, ahead of centrist KO, which had 31.9%. The other groupings in government, the Christian conservative Third Way and the Left scored 13.5% and 6.8% respectively.
"The unsinkable PiS ... However you look at it, it's a substantial defeat for the image (of Tusk's party)," Krzysztof Izdebski, policy director at the Stefan Batory Foundation think-tank, wrote on X.
Tusk swept to power following a national election in October on promises to reverse democratic backsliding, boost the rights of women and minorities and repair ties with Poland's Western allies that had become strained under the previous government.
PiS came first in that election but lost its majority, and of late the party and its allies have been plagued by infighting.
Brussels is closely watching how Tusk's coalition fares as it faces European Parliament elections in June, while Europe-wide opinion polls suggest the populist right will perform well.
STRONG IN WARSAW
Speaking shortly after exit polls were published, Tusk said his group's failure to emerge on top was partly due to strong turnout in PiS heartlands in eastern Poland and throughout the countryside.
One place where his party did very well was in Warsaw, where the incumbent mayor Rafal Trzaskowski, 52, secured re-election without having to go into a second round of voting.
Political observers said the strong showing in the capital bolstered his prospects for being nominated to run for president in an election due next year.
That job is seen as key for the Tusk government's ambitions to reform the judiciary, which critics said had been politicised under PiS rule, and to diminish the influence of PiS' conservative ideals on public life.
"Now KO will have to convince Trzaskowski (to run for president) instead of him having to ask for support," said Rafał Chwedoruk, a political scientist at Warsaw University.
The conservative PiS ally, Andrzej Duda, 51, has pledged to oppose much of Tusk's agenda and last month vetoed a bill to ease women's access to emergency contraception.
Trzaskowski lost to Duda in the 2020 presidential election.
A second round of voting is due in two weeks in areas where mayoral candidates failed to win more than 50% of votes.
Voters cast ballots to choose members of 16 regional assemblies - whose composition roughly reflects support for the main parties - as well as thousands of local councillors and mayors.