(Adds background, opposition quote)
By Jan Lopatka
PRAGUE, Oct 25 (Reuters) - Czech Prime Minister Mirek Topolanek was considering quitting as his right-wing party's chief on Saturday after he suffered a big defeat from the leftist opposition just ahead of taking over the EU presidency.
Topolanek's Civic Democrats (ODS) won just three seats in an upper house election on Saturday, final results showed, a result that angered Topolanek's party rivals and even raised doubts about the cabinet's future.
The leftist opposition Social Democrats took 23 seats, prompting them to call on the government to resign and hold an election in mid-2009, ahead of regular polls due in 2010.
Topolanek, facing a tide of anger from many in his party who attributed the loss to his unpopular economic reforms, said he would consider giving up his party seat at a Civic Democrats' congress in December.
"This means for me personally that I will have to decide whether to run and defend my seat of ODS chairman," Topolanek said. He added that the government also needed to go through a period of "reflection", and possibly a reshuffle.
The Senate, which approves laws and treaties but can be overruled by the lower house, has 81 seats, and one third is up for election every two years. The Senate rout followed a similar Civic Democrats' loss in a regional election last week.
Topolanek had earlier said he could even see the possibility, albeit unlikely, his party may leave government following the party congress.
That would play into the hands of the Social Democrats, who have called for a government of non-partisan experts to take the country through the EU presidency.
"This victory opens the way for this government to leave," Social Democrat chief Jiri Paroubek said.
The Czech six-month EU presidency starting in January has already raised concern among some EU politicians who want stronger leadership at a time of financial turmoil, and a political crisis would further reduce the Czechs' clout.
RULING PARTY DIVIDED
Topolanek's main rival within the party, Prague Mayor Pavel Bem, said it should consider calling its congress earlier, but refused to say if he would run for the chairmanship.
Topolanek's rivals have close links with the strongly eurosceptic President Vaclav Klaus, who has often been at odds with Topolanek, a pragmatist in relations with the EU.
The Czechs are in no rush to adopt the euro and have not yet ratified the Lisbon treaty meant to streamline decision-making in the EU. Topolanek wants to have the treaty ratified before the presidency starts, despite opposition from Klaus.
Parliamentary support for the cabinet, which also includes two coalition partners, has dwindled from 100 to 98 out of the 200 seats after two defections last month, but it has been able to scrape through with the help of several independent deputies.
The opposition failed to overthrow the cabinet in a no-confidence vote this week, which showed the main threat to Topolanek comes from his own ranks.
The cabinet has managed to push through unpopular reforms which included welfare spending cuts, a sales tax hike, and introduction of fees in healthcare. These won some praise from economists but alienated the public, along with plans to host a U.S. missile defence base.