The two sides said that despite their different outlooks, they would club together to destroy Mr Dini, a decision that threatens to wreak chaos for months and throw the economy into disarray just when it has begun to impose some order on its debt-ridden public finances.
If their MPs follow party lines in the debate starting today, they will defeat Mr Dini by a narrow margin and thereby scupper the remainder of his policy programme, including the 1996 budget, which has been presented to parliament but has yet to be approved.
Its suspension would embarrass Italy as it prepares to take over the European Union presidency for six months from January and risk relegating it definitively from the premier league of European nations.
Traders on Italy's financial markets, sensing the storm, sold lire yesterday and pushed the Milan bourse's Mibtel index to its lowest level.
Mr Dini's seemingly thriving non-political administration has suddenly been sucked into the kind of venomous power games that used to plague the country in the darkest days of the old Christian Democrat-led political order.
The crisis erupted on Thursday, when Mr Mancuso, a career jurist whose attacks on anti-corruption magistrates had alienated him from public opinion and his own government colleagues, was defeated in a no-confidence motion - unprecedented for an individual minister - in the Senate.
He responded on the Senate floor with an attack on Mr Dini and President Oscar Luigi Scalfaro, accusing them of conniving at grave abuses at the Milan prosecutor's office. He had a text of his speech distributed to journalists. It contained more damaging allegations of undue interference by Mr Scalfaro. He also appealed against his removal to the Constitutional Court, which is due to discuss it today.
Mr Mancuso's performance struck a chord with Mr Berlusconi, who goes on trial in January on corruption charges related to his Fininvest business empire. The media tycoon's Freedom Alliance immediately subscribed to every one of the Justice Minister's accusations and accused Mr Dini of trying to get rid of Mr Mancuso for base political reasons.
A no-confidence motion swiftly followed but Mr Dini let it be known he had no intention of resigning without a fight in parliament.
President Scalfaro, meanwhile, rebuffed the accusations against him one by one and accused Mr Mancuso, through a spokesman, of spreading smears like a mafioso.
As though a full-scale battle between politicians and the institutions of state were not enough, Rifondazione Comunista then emerged with its own plans to get rid of Mr Dini, accusing him of exacerbating the gulf between rich and poor in his latest budget proposals and arguing that his interim mandate had gone far enough.
Rifondazione Comunista was unable to drum up enough signatures to present its own no-confidence motion, so it finally decided to tag along with Mr Berlusconi.
The vote is expected to take place on Thursday.
Mr Dini has faced no-confidence votes before, and won them because deputies finally understood the urgency of his attempts to solve Italy's economic problems and broke with party ranks in sufficient numbers to keep him in power.
This time, however, will be closer than ever. If Mr Dini loses, the President is likely to call general elections before Christmas to keep the political damage to a minimum.
But even if he wins, he will not be out of the woods. Mr Berlusconi has threatened to order his coalition to resign en masse, or at the very least to sabotage the budget when it comes up for debate. This is a crisis that will not go away.
Currency turmoil, page 20