Mr Yossi Beilin, the architect of the Middle EAst peace plan known as the "Geneva Accords", has been elected leader of the newly formed left-wing Social Democratic Israel Party (SDIP).
"The peace camp has had its say. This means yes to peace, yes to Geneva, yes to social justice and a welfare state," said Mr Beilin, a veteran politician who helped draw up the historic 1993 Oslo Accords with the Palestinians.
Mr Beilin (55) won more than 56 per cent of the 16,000 votes cast for the leadership, a bloc that includes the left-wing Meretz party and a party Beilin formed when he left the centre-left Labour Party 18 months ago.
He immediately called for Labour leader Mr Shimon Peres to work with him to "shorten the days of the government of Prime Minister Ariel Sharon".
Mr Beilin won praise internationally for a "virtual" peace treaty drawn up with Palestinian cabinet minister Yasser Abed Rabbo last year which "resolves" all disputes including borders, the fate of Jerusalem and Palestinian refugees.
Israeli officials have accused Mr Beilin of violating democratic norms by going behind the back of the elected government and drawing up the agreement. And Palestinian militants regard the initiative as capitulation.
The Geneva agreement has made little headway locally since it was signed in December. But political analysts see it as a catalyst for Mr Sharon drawing up his own unilateral plan mapping out a withdrawal from the Gaza Strip.