US Senator Arlen Specter has defected from the Republican Party to the Democratic Party.
Democrats now control 59 seats in the 100-member Senate and are close to a 60-seat majority that could give them control.
Democrat Al Franken, who won the election in November in Minnesota, would be the 60th but a Republican candidate is challenging the result in court.
Senator Specter, 79, said on Tuesday it was "a painful decision".
In a statement, he said: "I now find my political philosophy more in line with Democrats than Republicans."
He was a Democrat before first being elected to the Senate in 1980 and is due to seek re-election for a sixth six-year term in 2010.
The last time either party held 60 Senate seats was in 1978, when the Democrats had 61.