French President Nicolas Sarkozy could lose his majority in the Senate to a resurgent left in an election this Sunday for half the seats in the upper house, dealing him an electoral blow just seven months before a presidential election.

The opposition Socialist Party needs to take just 23 new seats from Sarkozy's ruling conservative UMP party, out of the 170 seats up for grabs on Sunday, for a victory the UMP's Senate head Gerard Larcher said would have "seismic" consequences.

A swing to the left in successive local elections in many of the regions where some 150,000 municipal officials will vote in the Senate election makes gains by the left inevitable.

Sarkozy has struggled with some of the lowest ratings of any recent French presidents, with the French public resentful of economic gloom, stubborn unemployment and a widespread perception that his presidential manner is too impulsive and showy.

"There is no doubt that there will be a gain in seats for the opposition," Bernard Accoyer, head of the lower house national assembly, told France Info radio this week.

Losing his upper hand in the Senate, where the UMP has 147 seats out of a total 343 to the Socialist Party's 115, would hobble Sarkozy, symbolically at least, in the last few months of his term, even though the lower house holds sway on legislation.

A left-leaning Senate would only have the power to delay parliamentary proceedings but a defeat on Sunday would be a psychological blow for Sarkozy coming on top of opinion polls which consistently show the left would win a presidential election held today.

It would also mark the first ever swing left for the upper house since France's Fifth Republic was founded in 1958.

"I don't know where it would be on the Richter scale, but it would be pretty seismic at any rate," Larcher said of the prospect of a UMP defeat.

There is no major legislation outstanding that a left-wing Senate could hold up, 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.

Otherwise, 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 21 July, and the 2012 budget bill should pass without hitches.

In all, 170 Senate seats are up for grabs on Sunday, with results expected to be known by early evening. The number of seats in the upper chamber also will be increased on Sunday to 348, to reflect a rise in the French population.

Jean-Pierre Bel, head of the Socialist group in the Senate, said any rise in the number of left-wing senators would be a victory. In a Senate election three years ago for the other half of the chamber, the Socialist Party gained 21 seats.

Sarkozy is expected to announce sometime in November that he will run for a second term in next April's election, and is likely to face Socialist Francois Hollande as his chief rival.