By Robert-Jan Bartunek and Waverly Colville
BRUSSELS (Reuters) - Poland's prime minister will go head-to-head with other European Union leaders in Brussels on Thursday in a row over the reappointment of Donald Tusk, a Pole, as president of the European Council.
The clash, rooted in rivalry between Tusk and Jaroslaw Kaczynski, leader of Poland's right-wing ruling party, is casting a shadow over efforts to forge a new unity in the EU as Britain prepares to quit.
Diplomats confidently predict a second term for Tusk, a former premier of Poland who was appointed Council chairman in 2014 before his own centrist Civic Platform party was ousted by the eurosceptic Law and Justice party of his nemesis Kaczynski.
Polish Prime Minister Beata Szydlo's tactic on Thursday will be to pressure the other EU leaders to delay their decision, thereby possibly opening the way for an alternative candidate.
On arrival, before a meeting with German chancellor Angela Merkel, Szydlo spoke about defending a principle of national rights over "force".
"Those who disregard this principle, do not build unity but disunity," she told reporters, suggesting that Warsaw ought to have a special say on Tusk because he was himself a Pole.
But many other leaders made clear their impatience with Polish threats. French President Francois Hollande, whose Socialist allies did for a time favor replacing the conservative Tusk, said one country could not block the deal.
Even Viktor Orban, the right-wing Hungarian premier who is Kaczynski's closest ally in Europe, said he supported Tusk.
Yet Poland's intransigence means the reappointment could require hours of haggling. If Poland secures a delay, the top job will definitely not go to another Pole, diplomats said.
Joseph Muscat, the Maltese prime minister who will chair the session to decide Tusk's mandate, said he wanted a decision on Thursday. Talks should start at 5 p.m. (1600 GMT).
"One country cannot block the decision. There are very clear rules which we will follow," Muscat said.
With British Prime Minister Theresa May attending her last such summit before she formally launches the two-year Brexit process later this month, the remaining 27 EU leaders have bigger problems to worry about than the Council chair.
They will meet again on Friday, but minus May, to prepare for a "unity" summit to be held in Rome on March 25, the 60th anniversary of the treaty that laid the EU's foundation.
The row with Poland, the bloc's biggest ex-communist state, has highlighted a deepening split between eastern members reluctant to cede national freedoms to Brussels and the richer western states that want to deepen EU integration in the hope it can boost prosperity and security and thus stem the rise of Brexit-inspired eurosceptics.
Talk of a "two-speed Europe" has intensified in recent months. Germany's Angela Merkel and other leaders say allowing states willing to pull closer together is crucial to the EU's survival, but wary easterners fear they could be left behind.
In theory, Poland's case against Tusk is doomed as leaders can simply reappoint him with an overwhelming majority vote.
But Muscat may try to forge some kind of consensus at the table. Tusk will leave the room as his fate is decided.
Kaczynski holds Tusk "morally responsible" for the death of his twin brother. Tusk was prime minister in 2010 when Lech Kaczynski, the then-Polish president, was killed in an air crash in Russia. Inquiries in both countries blamed pilot error.
In a letter to fellow leaders, Szydlo said Warsaw wants Tusk out because he has criticized government policies back home.
Tusk is concerned that Kaczynski is undermining Polish democracy, a view shared by others in the EU, but Szydlo framed her objections to his reappointment in terms of protecting sovereign national powers from Brussels.
Thursday's talks should see agreement on pressing ahead with new free trade pacts despite "protectionist tendencies" elsewhere - a reference to European concerns about new U.S. President Donald Trump.
Over dinner, leaders are due to pledge continued support - and possible EU and NATO membership - to western Balkan states where they are worried about what they see as the anti-EU influence of Russia.
The leaders will also review plans to curb illegal migration from Libya to Italy. Arrival figures are already higher this year than in 2016.
(Additional reporting by Lidia Kelly in Warsaw and Philip Blenkinsop, Alastair Macdonald, Jan Strupczewski, Gabriela Baczynska and Farah Salih in Brussels, Writing by Alastair Macdonald, Editing by Angus MacSwan)