Florence mayor Matteo Renzi is poised to win the nomination to become Italy's youngest-ever prime minister after Enrico Letta was forced to resign by their centre-left Democratic Party.
President Giorgio Napolitano held a day of formal consultations with political leaders on Saturday that proved more complicated than expected and analysts said Renzi could be asked to form a new government only on Monday, the AFP news agency reported.
"A happy ending is not guaranteed," said Angelino Alfano, leader of the New Centre-Right party, a minor partner in the current coalition whose votes could prove crucial to the formation of a Renzi government.
"We can't finish in 48 hours," Alfano said after meeting with Napolitano, adding that he would refuse to ally with Renzi if the government proved "too leftist" and would then push for early elections.
Nichi Vendola, leader of the opposition Left, Ecology and Liberty party, ruled out taking part in a Renzi coalition and said the new government would be tainted with the "original sin" of Renzi's underhand tactics.
The anti-establishment Five Star Movement, meanwhile, protested at what it called an undemocratic power grab by Renzi and boycotted the talks, saying Italians should be allowed to choose through elections.
Letta stepped down on Friday after the Democratic Party approved a motion calling for a new government proposed by the 39-year-old Renzi, elected to the party leadership in December.
Renzi will be the European Union's youngest prime minister if he succeeds and has promised radical reforms to combat rampant unemployment, boost growth and slash the costs of Italy's weighty bureaucracy.
Opinion polls show Renzi is popular, mainly because as someone with no experience in national government or parliament he is seen as a welcome breath of fresh air in Italy's discredited political system.
But surveys also indicate that most Italians would have preferred early elections and are opposed to what critics dismissed as a "palace coup" engineered by Renzi, following weeks of bitter feuding with Letta.
"Renzi is a few strides from the finish line in his quest for the premier's chair, but he has not managed to unite the country behind him," said Stefano Folli, an analyst for the business daily Il Sole 24 Ore.
"There is a slight scepticism in the air," he said, adding that Renzi's promise to govern until the next scheduled elections in 2018 was "not very credible" given the fragmentation of Italy's political scene.
Investors are betting on a Renzi government pushing through key reforms, however, with stocks rising as Letta resigned and Moody's ratings agency improving its outlook for Italy from negative to stable.