Outgoing Prime Minister Silvio Berlusconi's conservative coalition was set to reject Mr Prodi's candidate Giorgio Napolitano, an 80-year-old former member of the European Parliament, seeing him as too left wing.
Mr Napolitano emerged as Mr Prodi's compromise candidate late Sunday (local time) after a frenzied round of party meetings designed to break a deadlock.
That meant Mr Prodi dropping his preferred choice, 57-year-old Massimo D'Alema, after objections from the centre-right over his communist past.
Instead, the centre-right has proposed Mr Prodi's coalition provide a list of four candidates, former prime ministers Giuliano Amato and Lamberto Dini, the newly-elected Senate speaker Franco Marini and former EU Commissioner Mario Monti.
Both Mr Berlusconi's Forza Italia and close allies the Northern League rejected Mr Napolitano, and the UDC Christian Democrats were to hold a party meeting to discuss their tactics before the vote, which could continue for several days.
"We will vote no," said Maurizio Gasparri, a leading member of the National Alliance, the other party in the outgoing government.
"We have proposed a group of names to be put forward and we expect the centre-left to put them forward. To provide us with only one name is not a way to win agreement," said Mr Gasparri.
Fair appointment
Mr Napolitano, like Mr D'Alema, is a member of the Democrats of the Left (DS), the former Italian Communist Party reformed after the fall of the Soviet Union.
As the biggest party in the incoming government, it has pressured Mr Prodi for a representative in one of the important institutional posts.
"The biggest party in the coalition has the right to present its candidate for the presidency given that it is not represented in the upper echelons of the state," said leading DS figure Luciano Violante. "And it would really be an anomaly if it was not."
Mr Violante rejected the centre-right's proposal because "there isn't one member of the DS among them."
Mr Prodi's Union coalition said Mr Napolitano - interior minister in Mr Prodi's first government in 1996 and a former parliament speaker - was a candidate who would fulfill the centre-right's desire for a personality with a "strong institutional profile".
Mr Prodi must wait for a new president to be elected before he can form a government, despite having won national elections a month ago.
The narrow margin of his victory means he has to bow to opposition pressure to nominate a candidate acceptable to both his own disparate coalition and the centre-right.
Some 1,010 Italian lawmakers, made up of members of both houses of parliament and representatives from Italy's 20 regions, are eligible to vote in the presidential election.
A two-thirds majority is required to elect a president in the first three rounds of voting, and a simple majority after that.
Mr Berlusconi has already said his coalition would draw out the vote by backing his own candidate, his outgoing undersecretary Gianni Letta, in the first three rounds.
This would mean that the election would go on until at least Wednesday (local time), further delaying Mr Prodi's accession to power.
President Carlo Azeglio Ciampi, 85, is due to step down at the end of his seven-year term on May 18.