By Pawel Sobczak and Chris Borowski
WARSAW (Reuters) - Polish Prime Minister Donald Tusk will unleash rivalries that could tear apart his party, the cornerstone of Poland's most stable government since Communism, if he leaves after the next election to take another job he has been tipped for.
Tusk's Civic Platform party matters to investors in Poland, eastern Europe's biggest economy.
Many say the alternatives would hurt the economy - either a government led by the right-wing opposition which quarrelled with Germany and Europe the last time it held power, or a shaky coalition of smaller parties liable to collapse.
The six year-old government, which Tusk leads with a slim majority in the lower house of parliament, has proved one of the most robust and least scandal-prone in eastern Europe.
But a mini-rebellion by parliamentarians from Civic Platform has shown the party's fragility and Tusk's critical role as peacemaker between rival groups. If he leaves after the next election in 2015 to take a job in Brussels - which some inside his party say is a possibility - there is no obvious candidate to replace him.
Interviews with senior members of the party, commonly known by its Polish initials PO, describe an organisation with two opposing factions - one liberal, one conservative - that co-exist in an uneasy alliance.
Their differences burst into the open in January when the conservative camp, led by Justice Minister Jaroslaw Gowin, voted against giving legal status to same-sex partnerships which Tusk himself had advocated.
Tusk castigated Gowin but managed to negotiate a truce and the party carried on with business as usual.
"People believe that I am not on one of the wings in PO and so far I have been able to convince people with different ideas in the Platform to respect our main, unifying theme," Tusk said in an interview this week with the Gazeta Wyborcza daily.
The question is how much longer he will fulfil that role.
German newspaper Der Spiegel reported late last year that Tusk is the front runner to take over as president of the European Commission when the incumbent, Jose Manuel Barroso, ends his second term in 2014. The possibility of a move to Brussels is frequently discussed in Polish media.
Tusk said he was not applying and would see out his term, which ends in 2015. The party insiders who spoke to Reuters said Tusk would lead his party into the parliamentary election that year. But there were circumstances after that in which Tusk would consider a European Union job.
Tusk's government is the first in Poland to last into a second term since the end of Communist rule in 1989.
It has a reasonable chance of being elected again. A survey this month by pollster CBOS put Civic Platform on 25 percent support, against 22 percent for Law and Justice, the main opposition.
Tusk, 55, personifies the new, steady style. An uninspiring public speaker, some Poles joke about his background in the Kashubia region around Gdansk, whose people are stereotyped by other Poles as dour peasants.
His air of managerial competence and control has helped attract investment. Foreign ownership of Polish sovereign bonds is near record levels. The economy is slowing sharply but still grew 2.0 percent last year compared with a contraction of 0.3 percent in the EU as a whole.
"I think people see him as a driver of reform and of free market policy," said Peter Attard Montalto, an economist at Nomura. Investors would treat it as a "moderate negative," if Tusk decided to leave, he said.
In a political landscape polarised between conservatives and liberals, Tusk's party is the only one able to pull in votes from both sides of the divide. It is a recipe for electoral success that also makes the party itself a fractious place.
While Gowin leads the conservatives, the liberal camp's informal leader is Grzegorz Schetyna, a former deputy prime minister under Tusk. They share the same pro-market, pro-EU economic policies but differ on issues such as gay rights, abortion, and in-vitro fertilisation.
Civic Platform insiders spoke to Reuters on condition of anonymity because they were discussing internal party affairs.
A senior liberal inside the party said Gowin's faction made a mistake by openly opposing same-sex unions.
But he said there was no risk of Gowin's group leaving. "Without the Platform it would be hard for them to exist. I would not over-estimate the importance of the group around Gowin. For the Platform, the vote in big cities, the liberal electorate, is much more important."
A leading member of the conservative camp also said Gowin's group would not leave, but had a different view on its importance.
"Naturally, I am fully aware of the fact that the liberal faction holds the majority in PO. But it is through the conservative faction that PO wins the votes of (the) conservative electorate."
Where both sides agree is on the crucial role of Tusk in holding the party together.
Possible successors for Tusk, according to analysts and people in the party, include Warsaw mayor Hanna Gronkiewicz-Waltz, Transport and Construction Minister Slawomir Nowak, and Foreign Minister Radoslaw Sikorski.
"But at the moment we don't see a politician that would have both public support and a strong position in the party," said Miroslawa Grabowska, a political analyst from Warsaw University.
(Additional reporting by Marcin Goettig; Writing by Christian Lowe; Editing by Erica Billingham)