French opposition Socialists claimed that they had won enough additional seats to give thema commanding majority in the Senate after Sunday's elections, dpa reported.
If confirmed by election officials, it would give Socialists their first majority in the Senate since World War II ahead of next year's presidential elections, when President Nicholas Sarkozy will be seeking reelection.
Socialist faction chief Jean Pierre Bel spoke of a historic change in the Senate elections and said the party had won an additional 23 seats in Sunday's vote.
Socialist Party head Harlem Desir said the outcome represented a resounding "slap against Sarkozy."
Sarkozy's Union for a Popular Majority (UMP)had dominated the upper house of French parliament since the Fifth Republic was declared in 1958.