Failing to form a government would further delay sorely needed economic and political reforms in a country where widespread corruption discourages investment, recession has devastated the job market and Italians are fed up with ever-higher taxes as the price for surviving the eurozone-debt crisis.
Letta, 46, a veteran lawmaker highly regarded by Italy's politically influential pro-Vatican centrists, needs to secure wide and reliable support in Parliament for an agenda that would balance measures for both austerity and growth.
Meanwhile, Berlusconi, who has been premier three times, is intent on furthering his political comeback while protecting his business interests and himself in various criminal trials.
Roberto Maroni, a Berlusconi ally, said his Northern League forces won't join the government. He told reporters that Letta was "neither optimistic nor pessimistic" about his prospects to form a government.
Letta is very "aware that if its attempt fails, we go to early elections," Maroni said.
It was with Letta's meeting with lawmakers from the anti-establishment, grassroots 5 Star Movement, which made a stunningly successful first foray into national politics, that the premier-designate acknowledged uncertainty about his ability to form a coalition.
Parliament's third-largest bloc, the movement, led by comic-turned-political agitator Beppe Grillo, won't join a government, but might support Letta on some measures.
Letta's publicly squabbling party splintered after it won control of the Chamber of Deputies, but not of the Senate. Not only must the premier-designate form a government, he must stop his own party's implosion to ensure its lawmakers will close ranks on the government's reform platform.
Democratic Party chief Pier Luigi Bersani resigned last week after back-to-back humiliating failures. Although his party was the largest vote-getter in February's election, he failed to line up a coalition government after President Giorgio Napolitano asked him to try to do so. Next, traitors in his own party sabotaged his high-profile choice, Romano Prodi, a former European Commission president, to succeed Napolitano, whose term in office was expiring.