Democrat Al Franken has been sworn in as US Senator for Minnesota, potentially giving his party 60 votes in the chamber.
Mr Franken narrowly trailed his Republican opponent on election day last November, but was declared the winner last week after a recount and legal battle.
Mr Franken first rose to prominence as a satirical comedian. His career included appearances on Saturday Night Live. He later became a best-selling author and a radio host.
Having 60 of the 100 seats in the Senate could help the Democrats defeat delaying tactics by the Republicans. Control of three-fifths of the Senate is needed to overcome a filibuster.
However, two of the votes Democrats rely on come from independents. In addition, two prominent Democrats, Senator Ted Kennedy and Senator Robert Byrd, have missed a number of votes because of illness.