By Pawel Sobczak and Christian Lowe
WARSAW (Reuters) - The Polish government's majority in parliament fell to two seats on Monday with the departure of one of its most senior lawmakers, leaving Prime Minister Donald Tusk's grip on power at its most fragile since he took office six years ago.
Tusk's rule is not under immediate threat, but any further erosion of his support in parliament could lead to the collapse of his coalition and early elections, with hard-to-predict consequences for central Europe's biggest economy.
Jaroslaw Gowin, who unsuccessfully challenged Tusk for the party's leadership in a contest last month, became the second member of parliament to depart Tusk's Civic Platform party in the past two weeks in arguments over policy.
A first test for Tusk's reduced majority will come when the 460-seat parliament votes on a government law on transferring some assets from private pension funds to the state. Opponents, including some in Tusk's own party, say it amounts to a nationalization.
Gowin said the pension reform triggered his departure, but it was preceded by months of wrangling. Gowin led a conservative faction within the Civic Platform that believed Tusk too liberal on issues such as abortion and same-sex partnerships.
"Today we have reached a threshold, beyond which loyalty to the party will be in conflict with loyalty to the Polish people," Gowin told reporters. "Remaining in the party would be contrary to my conscience."
Fragile governing coalitions are not unusual for the volatile ex-Communist states in eastern Europe.
However, Poland under Tusk appeared to have bucked the regional trend. It established a robust government and won praise from markets for its stable, pro-market policies.
That stability helped the Polish economy escape recession after the 2008 global financial crisis - the only European Union economy with that distinction. Its growth helped pull its wealthier European neighbors out of their downturn.
In the past few months, Tusk's government has lost its aura of invincibility. A short-lived economic slowdown, and party infighting, resulted in the Civic Platform's opinion poll rating falling behind the opposition Law and Justice party.
The zloty currency was largely unchanged on Gowin's departure. But analysts said any sign that the government cannot get financial legislation approved in the Sejm, or lower house of parliament, would cause nervousness in the markets.
"The smaller the majority in parliament, the bigger the risk is," said Piotr Bielski, senior economist at Bank Zachodni WBK, a unit of Santander.
Gowin is one of a group of three Civic Platform legislators, all members of the informal conservative camp, who were facing disciplinary action for voting against the government on a law that fractionally eased budget spending limits.
John Godson, who was born in Nigeria and is Poland's first black member of parliament, quit last month. The third member of the group, Jacek Zalek, may announce his departure soon, party insiders say. He declined to comment on Monday.
That would leave Tusk with a majority, on paper, of one vote in parliament. Political analysts say that in reality Civic Platform party managers can usually bolster their support by persuading a handful of independents to back the government.
However, that leaves Tusk with only a small margin of maneuver that could quickly vanish.
Gowin has many more sympathizers within Civic Platform; some estimates put the number at several dozen. So far, none has revealed any intention of leaving.
A contested vote could move them to quit, for example when the pension reform comes before parliament, which will happen within the next two or three months.
A defeat on an important vote could bring down the government, forcing either a re-configuration of the governing coalition or, if that fails, an early election. The next election is scheduled for 2015.
One political analyst said an early election could actually favor Tusk, giving him more chance of securing another four years in office. He said last week he was prepared for an early vote if his coalition lost its majority.
"The key is to hold them fast," said Anna Materska-Sosnowska, of the Political Science Institute at Warsaw University. "The more time passes, the more support the Platform could lose. Political events will be accelerating now."
(Additional reporting by Karolina Slowikowska and Dagmara Leszkowicz; editing by Ralph Boulton)