Saad Hariri has powerful allies in Saudi Arabia and the West.
By Natalia Antelava
BBC News, Beirut
The pro-Western alliance, led by 39-year-old leader of the Sunni majority Saad Hariri, won a majority of seats in the June parliamentary election, beating the Iranian-backed opposition led by the Shia militant and political group Hezbollah.
Mr Hariri has failed, in more than 10 weeks, to form a government.
The young billionaire businessman Saad Hariri heads one of the largest business conglomerates in the Middle East and has powerful allies in Saudi Arabia and the West.
But he is best known for being the son of Rafik Hariri - Lebanon's former prime minister who was killed in Beirut in 2005.
The assassination, which altered the course of Lebanon's history, marked the beginning of Saad Hariri's own political career.
Rafik Hariri's heir emerged at the forefront of the campaign for justice for his father. Along with his supporters at home and in the West, he blamed Syria for the car bomb that killed his father and forty others.
Damascus denied any involvement, but the assassination sparked such outcry that Syria was forced to withdraw its troops, ending its 30-year domination of Lebanon.
But more recently Saad Hariri has softened his anti-Syrian rhetoric and following the parliamentary election in June, he vowed to work together with the pro-Syrian opposition led by Hezbollah.
"His rhetoric now is much more reconciliatory, and is very different from what it was two years ago, when he was clearly a divisive figure," says Rami Khoury, the director of the Issam Fares Institute for Public Policy at the American University of Beirut.
Prior to his political career, Saad Hariri was best known for his playboy lifestyle, but many say that over the years he has transformed himself from a rich and inarticulate young man into a much more seasoned and assertive politician.
"He has made mixed impressions, and he still has to prove himself, but I think he has shown himself as a smart man who is up to the task," says Mr Khoury.
Saad Hariri has met with the leader of Hezbollah, Hassan Nasrallah.
But the task is daunting - Saad Hariri will need to create a unity government in the country, which remains deeply divided along sectarian lines.
On the eve of his nomination as prime minister, Saad Hariri met with one of his main opponents, the leader of Hezbollah Hassan Nasrallah. At the end of their rare, four-hour meeting, the two men vowed to work together.
But like everything else in Lebanon, Saad Hariri's success, or failure, as the prime minister will depend largely on what happens outside Lebanon.
This tiny country has always been the battleground of regional powers.
Many here believe that the success and calm of the June parliamentary election is the direct reflection of the new dialogue between the old regional foes - Syria, which supports Hezbollah, and Saudi Arabia, which backs Saad Hariri.
Equally, the recent events in Iran, Hezbollah's biggest backer, have undermined the potential for a dialogue between Tehran and Mr Hariri's allies in Washington - and that too could have a real impact on what happens in Lebanon.