By Joanna Plucinska and Angelika Meczkowska
WARSAW (Reuters) - Poland's main opposition parties have formed two coalition blocs to vie for left-leaning and centrist votes ahead of a national election later this year, in the hope of denting the popularity of the ruling nationalist Law and Justice (PiS) party.
Critics say a PiS win could drive Poland deeper into conflict with the European Union over reforms to the judiciary as well as migration and environmental policy, while moving the central European country further away from liberal values.
PiS has remained popular in part due to generous social spending programmes.
With opinion polls showing PiS on course to win a new term in the election expected in October, opposition politicians have struggled to formulate a competing message, riven by divisions over issues such as abortion and gay rights.
A broad coalition of opposition parties on the left and centre-right lost to PiS in May's European Parliament elections. It broke up shortly after the vote, underlining the political divisions in Poland.
Leading opposition party Civic Platform -- once led by outgoing European Council President Donald Tusk -- said on Thursday it would join forces with two small, liberal groupings to compete in the national vote.
"We believe that only this new formula will stand a chance against PiS," PO's leader Grzegorz Schetyna told reporters.
The progressive Wiosna of Robert Biedron, Poland's first openly gay lawmaker, who advocates taxing the powerful Catholic Church, as well as two small left-wing groups, announced plans to form a separate coalition.
Many political observers in Poland have said fragmentation within the opposition has weakened its chances of overcoming PiS, drawing parallels with the strength of Hungarian Prime Minister Viktor Orban.
Like PiS, Orban's Fidesz party has built its popularity on opposing migration and advocating traditional Christian values, while squabbling with Brussels over democratic standards.
Whether the new opposition blocs can grab power may depend on their ability to galvanise voters -- turnout at Polish elections can be as low as 50 percent.
Some observers also say the leftist grouping could fail to win at least 8% of votes, the threshold needed to enter parliament. A similar scenario helped PiS win a majority four years ago, when a different left-leaning coalition won just over 7%.
If Civic Platform were to go into the elections alone, PiS would garner 43% of the vote while PO would get 26.1%, according to an IBRiS poll published on Wednesday. Alone, Wiosna could get 4.2%, the survey showed. Its coalition partners were not measured.
But while the split might weaken the chances of either coalition conquering PiS, it might lead to a stronger opposition overall in the next parliament, Agnieszka Kwiatkowska, a sociologist at SWPS University, told Reuters.
That could prevent PiS from securing its aim of a two-thirds majority in parliament that would allow the party to change the constitution and make it more difficult for PiS to push through further reforms.
"The opposition going in two blocs, a centrist one and a leftist one, could spur a greater mobilisation of leftist voters, and that could lead to more seats for the democratic opposition," Kwiatkowska said.
(Additional reporting by Alan Charlish and Ania Gavina, Writing by Joanna Plucinska, Editing by Justyna Pawlak and Catherine Evans)