Despite his status as Israel's elder statesman and an impressive cabinet experience that includes two stints as prime minister, Shimon Peres has never led his Labour party to election victory.
Peres (left) has in the past served under Sharon
He has in the past achieved power in times of political crisis - often as a senior member of a broad coalition government.
As Prime Minister Ariel Sharon is being challenged by his own Likud party over plans to withdraw from Gaza, Mr Peres has the opportunity to come into his own again.
The 80-year-old Labour leader is a survivor of Israel's turbulent political scene.
He lost elections in 1977, 1981, 1984, 1988, and 1996.
However Mr Peres has held numerous cabinet posts over the years - most recently serving as Mr Sharon's foreign minister in the last national unity government, in 2001-2.
His greatest successes have come on the international stage, and he was the guiding light behind Israel's first interim peace accord with the PLO in 1993 in Oslo.
Born 1923 in Poland
Emigrated to British-administered Palestine 1934
Member of parliament since 1959
Has held many cabinet posts since the 1970s - most recently foreign minister in 2001-2
Prime Minister in 1984 and 1995-1996
Labour leader since June 2003
Never led his party to Labour victory
A high point of Mr Peres' long career came in 1994, when he shared the Nobel Peace Prize in recognition of his role.
He first became prime minister in a national unity government in 1984, and took over the top job again after the assassination of Yitzhak Rabin in 1995.
In 2000 he failed in his effort to secure the ceremonial post of president, losing to the relatively obscure Moshe Katsav.
It seems that he has never convinced a majority of Israelis that he can deliver security along with peace with Israel's neighbours.
In June 2003, Mr Peres was elected interim leader of the Labour party, a few months after a crushing election defeat for Labour under Amram Mitzna earlier that year.
Since then he has extended a "safety net" in parliament to Prime Minister Sharon, enabling him to pursue a plan to disengage from Gaza and parts of the West Bank in the face of opposition from Likud.
However he has set conditions for joining another national unity government - in which he sought the foreign affairs portfolio again, but has been offered and accepted the role of deputy prime minister.
The conditions include negotiations with the Palestinian Authority Palestinian - which Mr Sharon has resisted - and a return to Israel's pre-1967 borders "with some adjustments needed for security and to find a solution for settlers".
For a man widely viewed as a dove in the context of Israeli politics, Mr Peres was hawkish on Israel's nuclear programme.
He has gone on record as admitting that Israel's nuclear facility at Dimona - the building of which he oversaw - was established as a deterrent, although Israel maintains an official policy of what is known as "ambiguity".