(Updates with Casini comments)

ByStephen Brown

ROME, March 28 (Reuters) - Italian opposition leader Silvio Berlusconi may have scored an "own goal" by not supporting an Afghan peace mission in a Senate vote which, instead of showing how weak the government is, split the centre right in two.

Berlusconi's coalition, ousted by Romano Prodi's alliance last year, looked closer to what the media dubbed a "divorce" on Wednesday when the Christian Democrats (UDC), who broke ranks in Tuesday's vote, asked to meet the head of state -- alone.

UDC leader Pierferdinando Casini, a centrist who has long been a thorn in Berlusconi's side, said he trusted opposition supporters would realise "that in the Senate, Berlusconi made a serious mistake -- an own goal" over the vote.

Whilethe UDC supported the government Casini met President Giorgio Napolitano to ask for Prodi's resignation as prime minister. Casini wants to show that despite backing the government on Afghanistan, he still firmly opposes Prodi, who would have won anyway without UDC votes.

"Italy needs a new government that brings stability to the country," Casini said, adding his party "will stay in the opposition".

Casini says Prodi should quit since the Senate risked paralysis without a strong majority. But as Prodi recently won a confidence vote, Napolitano is unlikely to accede now to the request.

"We could not abandon our soldiers abroad. No responsible centre-right leader in Europe would do that," Casini said of the vote to keep 1,900 Italian troops in Afghanistan on peacekeeping duties with NATO.

TACTICAL DEFEAT

Berlusconi and his old foreign minister Gianfranco Fini, supporters of Washington and the Iraq and Afghan campaigns, abstained in the Senate, which is equal to a 'no' vote.

They argued that with the Taliban campaign getting more ferocious, and Italians more vulnerable thanks to last week's swap of Taliban prisoners for a kidnapped Italian, the troops did not have the equipment or the mandate to defend themselves.

"Now Berlusconi will have to explain himself to our foreign allies and the Italian people," Prodi said on a visit to Chile.

The centre-left Union leader said the picture of a "united majority and divided opposition" flew in the face of persistent talk of rifts in his coalition since coming to power last May.

But columnist Massimo Franco wrote in Corriere della Sera that "while Berlusconi's tactical defeat is clear, both blocs come out of this looking weak. The Union is still at risk and hostage to the far left".

The prospect of a UDC divorce from the opposition worries Prodi's leftist allies, as it could herald a coalition shift to the centre to include the UDC and marginalise the communists.

"If the UDC becomes an organic member of the coalition, we will leave," said Italian Communist leader Oliviero Diliberto.

Talk has resurfaced of a long-term project between Casini and centrists in Prodi's camp for a future alliance excluding right and left from power.

But Foreign Minister Massimo D'Alema reassured leftists that "there will be no broader alliance, there is no plan to push the left to the fringes of the coalition". (Additional reporting by Francesca Piscioneri in Chile)

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