France's opposition Socialist Party is expected to make strong gains during Sunday's vote to fill half the government's Senate seats.

Polls are projecting a historic swing to the left just seven months ahead of presidential elections, delivering an early electoral blow to President Nicholas Sarkozy's UMP party.

The Socialist Party needs just 23 new seats out of the 170 up for grabs to secure a majority in France's upper house of parliament.

Mr. Sarkozy is struggling with some of the lowest ratings of any recent French president. In addition to criticism over his government's handling of the economy and unemployment, he is also embroiled in a corruption probe linked to arm sales.