Republican Scott Brown took over the seat of the late Massachusetts Sen. Edward Kennedy on Thursday, vowing to be an independent voice in a bitterly divided Senate.
"I can't promise I will be right in every vote I make. ... I will do the very best job I can," Brown told reporters.
Brown was sworn-in by Vice President Joe Biden at a Capitol Hill ceremony a week earlier than he originally planned, and just in time to plunge into a partisan fight over President Barack Obama's choice of a union attorney for a top labor job.
Brown's arrival in the Senate ends the Democrats' supermajority and gives the GOP 41 votes they can use to block President Barack Obama's agenda.
At a news conference, Brown made an assertion about the last economic stimulus bill that most economists would dispute.
"The last stimulus bill didn't create one new job," Brown said in response to a question about a jobs bill pending in the Senate. He added that the stimulus may have retained some jobs.
Republicans have questioned the way the Obama administration has counted jobs created or saved with stimulus money. But most economists believe new jobs were created even though unemployment rates rose.
Depending on how Democrats set the Senate's calendar, Brown's first vote could be against the confirmation of Craig Becker, a lawyer for the Service Employees International Union, to a seat on the National Labor Relations Board, the federal panel that referees private sector labor-management disputes.
Brown said he hasn't decided on whether to support Becker.
The Senate Health, Education, Labor and Pensions Committee approved Becker's nomination on a party-line 13-10 vote Thursday, sending it to the full Senate.
Republicans have held up Becker's confirmation for months, saying they fear he might use the post to make labor laws more union-friendly without congressional approval.
Brown, 50, a little-known former state senator, stunned the nation with his upset victory last month over favored Democrat Martha Coakley and put the 2010 midterm elections in play for a possible GOP takeover of Congress. Brown's win derailed Obama's health care overhaul and catapulted Brown onto the national stage.
On Wednesday, Brown said he wanted to move up the swearing-in so that he could participate in upcoming Senate votes. On Thursday morning, Massachusetts Gov. Deval Patrick certified the results of Brown's win, clearing the way for him to take the oath of office.
Conservative radio hosts and newspaper columnists had pressed for Brown to take office earlier.
Former Democratic Party Chairman Paul Kirk had been holding the seat temporarily after being appointed by Patrick after Kennedy's death.
Brown will fill the last two years of the late Kennedy's term. Kennedy held the seat for nearly a half-century before he died in August.
Associated Press writer Sam Hananel in Washington contributed to this report.
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