Carte Goodwin, a former top aide to West Virginia Governor Joe Manchin, was sworn in on Tuesday to temporarily replace the late U.S. Senator Robert Byrd, returning Democrats' control of the Senate to 59-41.
Vice President Joe Biden, as president of the Senate, administered the oath of office to Goodwin, who received a standing ovation from his new colleagues.
Goodwin, appointed by Manchin last week, is to serve until a special election is held in November when West Virginia voters will pick someone to hold the seat for the remainder of Byrd's term that ends in January 2013.
Earlier Tuesday, Manchin, 62, announced his anticipated Senate candidacy. As a popular two-term governor, he's seen as an early favorite.
Representative Shelley Moore Capito, 56, is considered the leading Republican contender. She's expected to announce her intentions within days.
Goodwin said when appointed that he was not interested in running for the seat. At 36, Goodwin is the youngest member of the current Senate, replacing Byrd, who at 92, was the oldest.
Democrats welcomed Goodwin to the U.S. Capitol and arranged to quickly put him to work helping them pass legislation to extend jobless benefits to the long-term unemployed.
Republicans have blocked the $34 billion measure repeatedly in recent days, but Democrats were expected to approve the legislation with Goodwin's vote.
With 59 votes in the Senate, Democrats are one short of the 60 needed to clear Republican procedural hurdles. They have been able to do so on a limited number of matters by getting one or two Republicans to cross the political aisle.