By Sophie Louet
PARIS (Reuters) - French President Nicolas Sarkozy's conservative government lost its majority in the Senate to the left on Sunday, officials said, in a historic defeat that deals him a blow just seven months before a presidential election.
For the first time since 1958, the right-dominated upper house swung to a left-wing majority as the body's membership underwent a major generational change of guard.
Early results from the indirect elections showed that left-wing candidates took at least 23 seats from the ruling conservative party, securing them an absolute majority.
The shift to the left, which UMP Senate leader Gerard Larcher had described as having "seismic" consequences ahead of a presidential election next April, drew howls of joy from left-wing supporters at a meeting in Paris.
"The 25th of September, 2011, will go down in history," said Jean-Pierre Bel, head of the Socialist group in the Senate, on LCI television. "The results of this Senate election represent a real comeuppance for the right."
The left's victory followed a series of wins by Socialist candidates in local elections in the same regions where tens of thousands of municipal officials -- empowered as so-called "super voters" in the Senate poll -- cast votes on Sunday.
A left-leaning Senate will not be able to derail Sarkozy's legislative plans but the loss of a longstanding bastion for the right is a symbolic setback especially when taken together with his persistently poor poll ratings.
Sarkozy has become slightly more popular in the past few months, but he remains one of the least well-liked presidents in post-war France and faces a tough battle for reelection in a two-round vote scheduled for next April.
French voters are depressed about their economic prospects, unemployment remains stubbornly high and a European debt crisis has invited intensive scrutiny of France's public finances.
Such worries have overshadowed Sarkozy's foreign policy victories, notably France's role in the toppling of Libya's Muammar Gaddafi.
Government spokeswoman Valerie Pecresse played down the election results, pointing out that Sarkozy retained a majority in the National Assembly.
"This is in no way a point of blockage for the government because, as you know, it's the National Assembly that has the last word."
"Local elections rarely favor the incumbent," she added. "We lost a number of towns and regions and mathematically, there was a progression for left-wing super voters."
Even if the lower house holds sway over legislation, an electoral defeat hurts Sarkozy in the final months of his term as he faces a tough battle for reelection against Francois Hollande, the most likely Socialist candidate.
"This victory creates a dynamic... If we could win the presidency of the republic after winning the presidency of the senate, that would be a good," Hollande said on LCI television.
There is no major legislation outstanding that a left-wing Senate could delay, but losing his majority there would bury Sarkozy's grand plan to get a budget-balancing debt rule written into the French constitution, a measure that could have been an anchor for France's AAA-rating.
Parliament already has approved an adjustment to the 2011 budget bill to incorporate a bigger bailout for Greece as agreed by euro zone states on July 21, and the 2012 budget bill should pass without hitches.
Of the 170 seats up for grabs in the election, the UMP had 147 seats out of a total 343 to the Socialist Party's 115 before Sunday's vote. Final results are expected later on Sunday.
The number of seats in the upper chamber will be increased on Sunday to 348, reflecting a rise in the population.
Sarkozy is expected to announce sometime in November that he will run for a second term in next April's election.
(Reporting by Catherine Bremer, Nicholas Vinocur, Emile Picy and Sophie Louet, writing by Nicholas Vinocur; Editing by David Cowell)