(Adds 1st vote, opposition, analysis and quotes)
ROME, Jan 23 (Reuters) - Italy's centre-left government struggled to stay afloat on Wednesday and even the president appeared to doubt that Prime Minister Romano Prodi, weakened by defections, could survive a confidence vote in the Senate.
As expected, Prodi won a first confidence vote in the lower house where he has a clear majority.
But with Prodi under pressure to step down before the vote in the upper house due on Thursday, the third biggest economy in the euro zone seemed doomed to prolonged political upheaval that will further delay reforms, just when a global slowdown looms.
Ordinary Italians, who have seen 61 governments since World War Two, seemed jaded by the prospect of more turmoil, while in worried markets, the spread between Italian government bonds and German bunds widened to levels not seen for 6-1/2 years.
A government source said President Giorgio Napolitano had advised Prodi to "review" the wisdom of facing the Senate, where he has lost his majority.
"After today it's 100 percent certain Romano Prodi will resign to avoid being beaten in the Senate," said right-wing National Alliance leader Gianfranco Fini.
"We will demand elections."
Prodi's once slim Senate majority has been erased by the defection of former justice minister Clemente Mastella's Catholic Udeur and the centrist Liberal Democrats.
Losing or drawing the vote would force Prodi to quit but the 68-year-old prime minister fuelled speculation he might resign earlier by visiting Napolitano -- a reminder of his tactical resignation last February meant to scare his allies into line.
On that occasion, Napolitano reinstated him.
Prodi's fate hangs on whether he can muster enough support in the Senate to compensate for the defections and for two coalition dissidents who have said they will vote 'no'.
Without his two-seat Senate majority, he relies entirely on seven unelected lifetime senators to survive. Some calculations show the best he can hope for is to win by one vote.
Giulio Andreotti, an ex-prime minister and honorary senator, said he saw "no alternative at the moment to this government, so I will vote for Prodi".
But Roberto Maroni, a deputy from the right-wing Northern League, said it would be a "minority dictatorship" if Prodi clung to power on the votes of unelected elder statesmen.
Conservative former prime minister Silvio Berlusconi, who was beaten by Prodi in 2006 elections, hopes a Senate defeat for Prodi will trigger snap elections. Opinion polls suggest his centre-right would win them by a clear margin.
But Napolitano might appoint an interim government to reform the messy electoral system before a new election is held.
"There is very little chance we will see a vote without the system being changed because both the government coalition and the opposition want to change it," said politics Professor Franco Pavoncello of John Cabot University in Rome. "After the change of the system we have to see what are the possibilities of really putting together a right-wing coalition with Berlusconi as the leader," said Pavoncello. "Still today he is certainly the leading force in the centre-right coalition."
Rome residents digesting the news in a cafe appeared used to such crises but tired of the inherently unstable voting system.
"We had hoped they would last a bit longer, but we're used to it," said Franco Fonte. "I just hope at least they can reform the electoral system." (Additional reporting by Francesca Piscioneri, Silvia Aloisi and Gabriele Pileri; Editing by Michael Winfrey)
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