* Opposition has chance to topple Czech government
* Cabinet could stay on for months even if govt defeated
* Early election not ruled out
By Jan Lopatka
PRAGUE, March 24 (Reuters) - Czech opposition Social Democrats have their best chance yet on Tuesday of toppling Prime Minister Mirek Topolanek's minority centre-right administration, midway through the country's EU presidency.
Ousting the three-party cabinet, made more likely by defections from Topolanek's camp, would jeopardise policymaking in a severe economic downturn and threaten efforts to ratify the EU's Lisbon treaty.
Even if forced out by an opposition victory, Topolanek's team, long hampered by its weak standing in parliament, would stay in power until politicians hammer out what to do next, which could take weeks or months.
Topolanek is seeking support from two deputies who have defected from his junior coalition partner, the Green Party, to squeeze through the vote, but the outcome is in the balance.
He said he would seek another mandate from President Vaclav Klaus if the cabinet falls, but did not rule out early polls.
"In case the government does not win ... tomorrow, and if it is not possible to form a new cabinet without support from the Communists, then the Civic Democrats clearly support the fastest possible way toward an early election, as soon as the summer of this year," Topolanek told reporters on Monday.
Social Democrat leader Jiri Paroubek said he wanted the cabinet to stay on, even after resignation, until June to avoid changing the team during the country's EU term.
He said a government of non-partisan experts should then take over and lead the country toward and early election in the autumn or next spring. Regular polls are due in mid-2010.
GOVERNMENTS UNDER STRAIN
The Czech vote comes just days after Hungarian Prime Minister Ferenc Gyurcsany said he would step down and after governments fell in Iceland and Latvia under the strain of the economic crisis.
While the Czech political turmoil is not as directly related to the crisis, the economy has suffered from a slump in exports, and figures out on Monday showed industrial output fell by 23.3 percent in January. But banks have held strong, the public has been quiet and Czechs are not heavily exposed to foreign debt.
The cabinet has suffered instead from factional infighting and has been wobbly ever since it was appointed in January 2007, with just 100 seats in the 200-seat lower house, now reduced to 96 -- some of them unsure -- compared with 97 opposition seats.
The opposition needs 101 votes to overthrow the cabinet.
Two independents, formerly from a eurosceptic wing of Topolanek's party, have been increasingly hostile to the cabinet, in part due to plans to ratify the Lisbon treaty, meant to make the EU more flexible after its eastern expansion.
The upper house, or Senate, where many Civic Democrat Senators oppose the pact, may feel less obliged hold Topolanek's line defending ratification if the government is ousted.