Quite apart from the votes he will garner, Mr Dini's presence has given the centre-left a anchor of respectability. Thanks to his past at the IMF and the Bank of Italy, and his record as a prudent fiscal manager in government, Mr Dini has the confidence of the financial markets and Italy's EU partners. He is also light-years away from the left's communist tradition, which still scares many voters.
But what is he really up to? Has he joined the left in an ideological change of heart, or is he just using them in a longer-term plan to replace Mr Berlusconi as leader of the right?
Members of the mainstream left are terrified they are about to be betrayed. If tomorrow's result is inconclusive, they fear Mr Dini might be tempted to take away seats the centre-left has offered him and try to form a separate governing coalition supported by centrists and disenchanted Berlusconi acolytes. It is a fear that Mr Dini and his newly founded political party, Rinnovamento Italiano, have tried to dispel. "It is a very complex situation, and much will depend on the outcome of the vote," said Mario D'Urso, junior minister for foreign trade in Mr Dini's government and a candidate for the Senate.
In an interview with the Independent, Mr Dini said that the centre-left was "the only viable alliance for this election and for the next government". But he indicated that in future he would be willing to govern with "whoever displays the best leadership" for Italy.
Although not a candidate at the last elections, Mr Dini joined the Berlusconi government as treasury minister in a programme to purge Italy of its statist past and create a Thatcherite free-market economy. He now rejects the right-wing label, calling himself a centrist.
On becoming prime minister, he did not actively seek alliances with the left but was given no alternative when Mr Berlusconi unexpectedly turned against him. Since then, Mr Dini's disenchantment has been less with the right than with the personalities of Mr Berlusconi and his reformed neo- fascist ally, Gianfranco Fini.
Mr D'Urso, a former president of the merchant bank Lehman Brothers, said the prime reason for Mr Dini's entry into politics was to protect the interests of the financial markets. Certainly, Mr Dini's dealing with the leadership of the so-called "Olive Tree" coalition have been calculating rather than genuinely warm: so successful was he in negotiating safe seats into which to parachute his candidates that he ended up with two more than he could fill.
One of the 38 sitting parliamentarians deselected to make room for the Dini bandwagon, speaking on condition of anonymity, said he and his friends felt like "lambs to the slaughter", obliged to support candidates they dislike for whom they have now sacrificed their careers.
Allowing for upsets, Mr Dini can count on around 35 parliamentarians in the first-past-the-post part of Italy's hybrid electoral system, plus a handful more if he jumps the 4 per cent barrier required to win seats in the proportional part of the contest. That will almost certainly make him the pivot upon which any future government must rest.
"If he doesn't betray us now, he almost certainly will in the future," said one left-wing politician, also requesting anonymity. "We just have to hope it will be later rather than sooner."