To call it a late-flowering love affair might be exaggerating, but there is definitely an attraction of mutual interests. With tensions growing within Italy's centre-left coalition government, the PDS seems to find it easier to talk to Mr Berlusconi and his party, Forza Italia, than it does to its own allies.
This week, under the influence of the PDS's canny leader Massimo D'Alema, the government cut what seemed to be an outrageous deal with Mr Berlusconi, guaranteeing him the right to continue running his monopoly on private television for the next six months, even though the Constitutional Court has deemed it to be illegal. Ostensibly the reason for the deal was to give parliament time to draw up new legislation that would modernise the whole of the broadcasting sector, including cable and satellite.
But that on its own does not explain why Mr Berlusconi was so scrupulously consulted on the matter, and a blind eye so glaringly turned to the blatant conflict of interests. The nub of the matter is that the PDS is finding it ever harder to keep the governing coalition together. The so-called "Olive Tree" stretches from the communist hard-left to the free-market right, an impossible clutch of customers to keep satisfied simultaneously when it comes to such key matters as the budget-cutting measures necessary to qualify Italy for European monetary union.
The 1997 budget, which has almost completed its passage through parliament, was sealed only by making big concessions to the left, and keeping public spending cuts to a bare minimum. If, as an IMF forecast has predicted, the government comes under pressure to slash the budget further next spring, some part of the coalition is bound to give way.To stave off this looming crisis, the PDS is courting allies further afield. Mr Berlusconi is certainly not about to join the government, but he can be useful to Mr D'Alema in other ways. First, he can agree not to filibuster parliament in its efforts to push through a backlog of important legislation.
Secondly, he can cooperate in overhauling the constitution to make Italy easier to govern. Further down the road, there might be room for further negotiation: support for further budget-cutting measures from Forza Italia, perhaps, in exchange for some kind of amnesty exonerating Mr Berlusconi from the various charges of corruption and business malpractice that he is facing. This may not be the politics of high principle, and indeed it is infuriating a minority of left-wingers including some members of the PDS. But it is a mark of the widely-acknowledged tactical brilliance of Mr D'Alema.
So far, Mr Berlusconi is playing along partly because it suits his own personal interests, and partly because he thinks he might yet be able to outwit Mr D'Alema. His allies are working hard on the hypothesis that the present government might fall next spring, and that a cross-party alliance cutting out both the far left and the far right could then take Italy into Europe and the next general election.
What is striking in all this is the erosion of the boundary between government and opposition - harking back to the old days of Christian Democrat hegemony in Italy. The notion of defeating one's enemy by inviting him in seems to be an enduring one in this country, and one that could yet save Mr Berlusconi from an ignominious exit from public life.