France's opposition Socialist Party is claiming victory in elections to fill half the government's Senate seats, delivering an early electoral blow to President Nicholas Sarkozy's UMP party.

Although official figures are not yet public, the Socialist Party announced that it had won the 23 seats needed to secure a majority in France's 348-member upper house of parliament. About half the seats — 170 — had been up for grabs.

The historic swing comes just seven months ahead of presidential elections. Though the Senate vote has no direct bearing on next April's poll, the defeat signals broader voter discontent with Mr. Sarkozy's party.

The French leader 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, Mr. Sarkozy is also embroiled in a corruption probe linked to arms sales.