Mr Casini and his Union of Centre Democrats are already semi-detached from Mr Berlusconi's alliance.
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Its leader, Pier Ferdinando Casini, says he will not help make either main candidate prime minister.
UDC's leader, Pier Ferdinando Casini, who had dreamt of holding the balance, will dream on.
Negotiations for cabinet slots to replace the Fini faithful will begin with the Catholic moderate, Pierferdinando Casini.
"In life you win and you lose, " said Pier Ferdinando Casini, whose centrist movement allied with Mr. Monti for the elections.
Pier Ferdinando Casini, the leader of the third-biggest party in Mr Berlusconi's government of 2001-06, gave his speech a rapturous welcome.
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But the UDC's leader, Pier Ferdinando Casini, had pledged never again to sit in a cabinet presided over by Mr Berlusconi.
Mr Casini, however, will be a tough bargainer and Mr Berlusconi's existing senior coalition partner, the Northern League, emerges stronger than ever.
"Andreotti is dead, but the Christian Democrats are alive, " Pierferdinando Casini, a former Christian Democrat and centrist politician, said in a television interview.
And it could be objected that Mr Casini's plan for a grand coalition to keep in power an unelected prime minister is democratically questionable.
UDC's best hope, the speaker of the Senate, Pier Ferdinando Casini.
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Mr Casini and many of his followers are former Christian Democrats.
That would not be easy if he were linked to old school, professional politicians such as Gianfranco Fini of the National Alliance or Pier Ferdinando Casini, a Christian Democrat.
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Both of Mr Berlusconi's senior lieutenants, Gianfranco Fini of the National Alliance and Pier Ferdinando Casini of the Union of Christian Democrats, have said as much in recent weeks.
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The head of the Future and Freedom (FLI) party, Gianfranco Fini tweeted that the Monti coalition opened up a "prospect of renewal" while Christian Democrat leader Pierferdinando Casini said it was not so much "a personal party but a hope for Italians".
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