The Socialist Party said that, with its Communist and Green allies, it had won enough seats to give the left control of the upper house for the first time in French history, a stepping stone towards a presidential win.
"Nicolas Sarkozy will go down in history as the president that lost the right its majority in the Senate," declared Francois Hollande, favourite to win the Socialist Party's nomination to run against Sarkozy next year.
"In a way it's like a premonition of what will happen in 2012," he said.
Sarkozy's prime minister, Francois Fillon, admitted the right had suffered from its divisions and that the left had made a "strong breakthrough".
"The moment of truth will come next spring. The battle begins tonight," Fillon said in a statement, calling on the right to unite behind Sarkozy's government in time to turn the tide before the late April vote.
Right-wing parties have controlled the Senate since the Fifth Republic was founded in 1958, and Sunday's flip to the left could break the already weak momentum of Sarkozy's unannounced re-election drive.
Before the vote, outgoing speaker Gerard Larcher had admitted to AFP that if he was defeated by the left it would be a political "earthquake" and "the preparations for the presidential election would be singularly changed."
The historic Senate victory also opens the door to a possible Socialist hat-trick, given that opinion polls suggest the left will win next year's presidential and parliamentary elections.
"For the first time, the National Assembly, the Senate and the president could be from the left, which would give them serious weight if they decided to modify the constitution," said political scientist Bruno Jeanbert.
The Senate is not chosen by universal suffrage but by a "super-electorate" of elected officials -- around 72,000 mayors, local and regional councillors, voting for figures on the basis of regional lists.
Around half the seats in the 348-strong house were up for grabs in the poll, and the left needed to add only 22 more seats to win a majority.
With many centrists, independents and non-party figures in the upper house, it might be a few days before the exact division of forces becomes clear.
The outgoing speaker, UMP stalwart Gerard Larcher, said before the vote that he was confident of maintaining at least a six- to 12-seat margin to win re-election to his post on October 1st when the new chamber meets.
But Bel now expects to gather enough votes to unseat him.
The right did not initially admit defeat, with results still coming in, but the mood in Sarkozy's camp was decidedly more sombre.
Budget Minister Valerie Pecresse said she regretted the result and said she was "sad" for Larcher and his team.
Claude Goasguen, a UMP lawmaker from Paris, admitted his party had had a bad day in the capital. "We have to take on board the consequences quickly," he said, calling for a root-and-branch renewal of the Paris party.
And the mood in the country appeared to have already had direct political consequences, with Pecresse confirming the threshold for top rate income tax might be brought lower.
The Senate vote has no direct bearing on next April's presidential poll, which will be open to all French voters and conducted over two rounds, the second a head-to-head run-off between the best placed candidates.
But defeat is an ill omen for Sarkozy, whose party is already nervous about his low poll ratings and the ongoing economic and financial crisis.
Sarkozy has attempted to play on his foreign policy credentials as the current leader of the G8 and G20 great power blocs and the main foreign champion of the Libyan revolution that toppled Muammar Qaddafi.
But whatever glory he may have picked up on the international stage has been drowned out at home by the implication of his closest allies in a series of high-level corruption and party-funding scandals.
Meanwhile, unemployment remains high and France's financial sector has found itself under attack on the markets, where traders fear its banks are overexposed to risky Greek and Italian debts.