First of all we must remind that the extremely low democratic quality of the Italian electoral law, that requires 20% of votes for any coalition to be able to be represented in that mockery of Parliament, radically corrupts the democratic representativity of the system, leaving out the real Left. This authoritarian distinction between party and coalition lists is also present in the Greek electoral system, what has forced SYRIZA to become a party in order to hope for electoral victory within the system (when originally and logically they were a coalition of multiple forces, some of which preferred to stand aside). Of course there are many other tricks to distort popular representation and favor the bourgeois twin party system like Anglosaxon "winner-takes-all" districts but... another day.
Then we must recall that the Democratic Party (PD) in Italy comes from the Left Democratic Party (PDS), which in turn comes from the once very combative and highly popular Italian Communist Party (PCI). The Bersteinian bourgeois decadence of this once communist party has no parallels in the recent history of Western Europe: once carefully kept out of the system by a coalition of bourgeois parties, led always by the Christian Democracy, sponsored by the Mafia and NATO, it first became "Eurocommunist" as reaction to the military invasion of Czechoslovakia, then Socialdemocrat after the collapse of the late Stalinist system in Eastern Europe, adopting the name of PDS and finally merely Liberal Bourgeois with the US-mimicking name of Democratic Party.
However the grassroots of the party have not changed that much yet and they feel betrayed by their support of Mario Monti's perpetual exception semi-colonial government and now again by the flirting with Berlusconi's PDL in the name of stability and what-not, what resulted yesterday in the re-election of ailing right-wing President Giorgio Napolitano.
From Struggles in Italy (emphasis mine):
This situation has already taken its toll on the Democratic Party. On April 19, Pierluigi Bersani, Secretary of the PD, resigned after the failure of his proposed ‘mediation’; so did Rosy Bindi, former Christian Democrat and current president of the Party’s General Assembly.
The high ranks of the party are shaking under the rage of the many rank-and-file members of the Party who carried hopes of a political renewal. Many old-school militants have publicly announced their disappointment with a party that systematically refuses to listen to its base of supporters. Meanwhile, old hawk of the PD Massimo D’Alema (who briefly lead the Executive in 1999) was spotted calmly strolling in Rome, and many see his shadow in the disastrous outcome of the PD-PDL mediation. Finally, this situation opens a breach in the already fragile centre-left electoral cartel, with SEL publicly walking out of the alliance to give open support to the M5S candidate.
(...) this solution seems to mark the end of the Democratic Party, undermining its already compromised political representativeness. Once again, the left failed to achieve real change but fell prey to its internal divisions, promoting an old name that failed to satisfy its own core of activists and members.
What now? Either the twin party Berlusconi-PD agree in another Monti-style perpetual emergency joint cabinet or they agree to call new elections, elections in which the PD may easily collapse, yielding power again to the corrupt gerontocracy of Bunga-Bunga Berlusconi.
An alternative may be the rise of billionaire buffoon Beppe Grillo but I seriously doubt that this populist movement without ideology can actually win elections, even if their rise clearly highlights the limitations of the current semi-colonial partitocracy.