"[Burris] is now the senator-designate from Illinois and, as such, will be accorded all the rights and privileges of a senator-elect," Senate Majority Leader Harry Reid and Sen. Dick Durbin of Illinois said in a joint statement after Senate lawyers determined that Burris' paperwork met Senate requirements to be seated.
Burris, appointed by embattled Illinois Gov. Rod Blagojevich, could be sworn in this week, giving Democrats at least 58 of the Senate's 100 seats.
The decision marks a significant reversal for Senate Democrats. They initially resisted the surprise appointment by Blagojevich, who is accused by federal investigators of seeking to trade the Senate seat for personal favors, amid fears that any appointee would be tainted.
"As governor, I am required to make this appointment. If I don't make this appointment, then the people of Illinois will be deprived of their appropriate voice and vote in the United States Senate," Blagojevich said of his decision to appoint Burris on Dec. 30.
Burris, in Chicago Monday while his lawyers met with Senate officials on Capitol Hill, told reporters he's "humbled and honored" to be Illinois' next junior senator, and is thankful for the opportunity to serve.
The Illinois Supreme Court ruled last week that under state law, Burris' appointment paperwork was valid and that it was up to the Senate to decide whether to seat him. But Reid and other Democrats had contended that it violated Senate rules unless the appointment was signed by both the governor and the Illinois secretary of state.
Burris, a former Illinois attorney general, defiantly showed up on the steps of Capitol Hill on Congress' first day in session, only to be told that due to the dispute over his standing that he would not be sworn-in with other new senators. Burris departed the Capitol surrounded by a swarm of media.
As the Burris appointment took on the feeling of a political sideshow, Senate Democratic objections to seating Burris weakened. Burris also steadfastly maintained that he promised Blagojevich nothing in exchange for the seat.
"If those allegations are proven to be true against the governor, then, you know, he will be in serious trouble. That in no way impacts me. There's no such thing as a tainted appointment by a chief executive who has the constitutional authority," Burris told the NewsHour of his appointment.
According to the New York Times, Burris's lawyers hand-carried to the Senate Monday an additional document bearing a state seal, an affirmation of the appointment and a mass-produced signature of the Illinois secretary of state along with the governor's original papers.
Last Friday, the Illinois House voted overwhelmingly to impeach Blagojevich, an action that sets up a Senate trial on whether he should be thrown out of office for his alleged actions surrounding the filling of Mr. Obama's vacant Senate seat and other accusations of abuse of power.