With Sharon's sudden collapse from a massive stroke, a grim-faced Olmert took the reins of power Thursday, trying to convey continuity but acknowledging in a special Cabinet session that the nation is in a "serious situation."
"Arik is not only a prime minister and a leader, but also a close friend to us all," Olmert said, referring to Sharon by his nickname. "This is a difficult time and we will stand together."
Sharon's wide chair at the center of the long Cabinet table was left empty, a sign of the temporary nature of Olmert's new position.
If Sharon remains incapacitated, Olmert will be the acting prime minister until elections in March. Sharon was expected to lead his political party, Kadima, to victory but now the race is wide open, reports CBS News correspondent David Hawkins.
While the 60-year-old Olmert will be a top contender in the race for prime minister, he will likely never be as popular as his mentor, placing the future of Kadima in limbo.
Olmert was Sharon's strongest supporter as the prime minister withdrew Israeli settlers and soldiers from the Gaza Strip in September. When other members of the hard-line Likud Party turned on Sharon because of the Gaza pullout, Olmert stood by the prime minister.
Olmert served as Sharon's point man, floating ideas before they became policy.
"Olmert can take credit for having sponsored disengagement before Sharon. He served as his vanguard in putting the plan to the public," said analyst Yossi Alpher.
Olmert was first elected at the age of 28 to parliament, serving as a lawmaker for seven terms, and holding several ministerial posts. In those years, Olmert was investigated several times for corruption, but was never convicted of wrongdoing.
Elected mayor of Jerusalem in 1993, Olmert held the post for 10 years, supporting Israeli moves to settle in Palestinian-dominated areas of the city. In 1996, he opened a tunnel along a disputed Jerusalem holy site, an act that sparked days of Israeli-Palestinians clashes in which 80 people were killed.
Recently, Olmert acknowledged he had erred.
He said he wished Begin were still alive so he could tell him that he was wrong and that pulling out of Sinai was the right thing to do.
A recent opinion poll found that Olmert, as leader of Kadima, could win by a small margin — but the situation is fluid and analysts said it is difficult to predict what would happen to the movement in Sharon's absence.
Olmert's closest rival for Kadima leadership, Justice Minister Tzipi Livni, would bring more votes for the party, according to the same survey.
Many of Kadima's potential voters, and many of the politicians who defected from Likud and the dovish Labor Party joined Sharon, not necessarily his party, analysts said.
"He (Olmert) is known to be a very shrewd politician and a very able guy. People always respected his intellect but maybe didn't like his personality," said Menachem Hofnung, a political scientist at Jerusalem's Hebrew University. "He's very outspoken, he's not known to be very warm to people."
As acting prime minister, the public could come to see Olmert as a leader, said analyst Gadi Wolfsfeld. "No one really loves Olmert, but no one really hates him, therefore he has a chance of convincing the public," he said.
Olmert was born in 1945 in the town of Binyamina in northern Israel. His military service included a stint as an officer in an infantry unit and later as a reporter for the Israeli army journal.
He earned his bachelor's degree in psychology and philosophy and later a law degree from the Hebrew University in Jerusalem and worked briefly as a lawyer before entering politics.
Married and a father of four, Olmert lives in Jerusalem and is known to be a die-hard fan of the Beitar Jerusalem soccer club.