Former Illinois Attorney General Roland Burris has been sworn in to the US Senate seat vacated by President-elect Barack Obama.
Democrats now have 58 seats in the 100-member Senate - their biggest majority since 1981.
In one of his final acts before leaving office next week, Vice President Dick Cheney administered the oath on Thursday to Senator Burris, who was appointed to the seat on 30 December by Illinois Governor Rod Blagojevich.
Mr Obama and Senate Democrat Party leaders initially opposed the appointment, calling it "tainted". Gov. Blagojevich has been charged with earlier trying to sell the seat to the highest bidder.
The Illinois Supreme Court has ruled the appointment valid.