Centre-left leader, Pier Luigi Bersani, has backed the front-runner, Franco Marini, as candidate for president.
Mr Bersani warns that he could put Italy at risk in a similar way to Greece.
"We are worried because disaffection and rage are spreading fast across Italy, " Mr Bersani said.
The Italian voters may deliver victory to the candidate of the left, Mr Bersani.
Bersani and Berlusconi are archrivals who have previously indicated they will not work together.
Mr Bersani's bloc has 340 seats - a comfortable majority - in the Chamber of Deputies.
Such a move would risk further alienating the voting base of Bersani's Democratic Party.
During an interview with Pier Luigi Bersani, I asked about his plans for growth.
Bersani, who narrowly beat former Prime Minister in the senate, did clearly take the lower house.
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Mr Monti could be offered a key post in a Bersani-led government under such a scenario.
Bersani, the popular party veteran, trounced Renzi in the primary by almost 22 points.
Bersani is currently leading the polls with Berlusconi trailing by 5 to 7 points.
Mr Bersani, the PD leader, claimed to discern a strategy in the most recent attacks.
Mr Bersani's first move after his victory was to visit workers in a textile factory.
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Mr Rutelli's latest move has been to table a rival bill to that of Mr Bersani.
During the French campaign Francois Hollande, rather like Bersani, had said "austerity was not the only option".
Mr Bersani's supporters complain privately that his disruptive adversary is merely seeking to shore up his credibility.
The head of state is expected to meet Mr Bersani on Thursday - the second day of talks.
Bersani has said "very explicitly that he will not use old political hacks in his Cabinet, " Walston said.
Nevertheless, "I don't think Bersani has a chance to put together a government, " said political scientist Robert D'Alimonte.
Pier Luigi Bersani, head of the Democratic Party, is due to hold a news conference at 1100 ET.
Bersani has vowed to continue with Monti's austerity measures and reforms, albeit with some adjustments, if he wins.
Siena -- the small Tuscan city where the banking giant is headquartered -- is a stronghold for Bersani.
Other measures put forward by Mr Bersani are less a matter of liberalisation and more about protecting consumers.
Mr Bersani's Democratic Party narrowly won the elections last month but lacks the upper house majority needed to govern.
As well as talks with Mr Bersani on Thursday, Mr Napolitano will consult with Mr Berlusconi and Beppe Grillo.
Mr Bersani was projected initially to win a majority in the Senate, where seats are decided region by region.
But since the elections, he has refused to work with either Bersani or Berlusconi, resulting in the political gridlock.
Pier Luigi Bersani was rebuffed by the anti-establishment Five Star Movement on Wednesday.
Bersani and Berlusconi are arch-rivals who have indicated they will not work together.
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