"It has been one of the greatest honors of my life serving the people of my district in the general court over the past 10 years as a state representative and a state senator," Brown wrote in the letter to the president of the Senate.
With Brown's seating, the Democrats' supermajority in the Senate dies. Democrats need 60 votes to avoid any Republican filibuster, but President Obama reminded his party members Wednesday that they still have a big majority.
"All that's changed in the last two weeks is that our party's gone from having the largest Senate majority in a generation to the second-largest Senate majority in a generation. And we've got to remember that," Obama said. "We still have to lead."
Brown's election seemingly ended the possibility of a Democratic-only vote for health care reform in the Senate.
"We need to basically go back to the drawing board," Brown said. "I'm hopeful there will be bipartisan negotiations."
White House press secretary Robert Gibbs said today Brown's win means bipartisanship is more important than before.
"I think it changes everybody's calculus as to taking responsibility for governing this country," Gibbs told reporters today. "Assuming that what is insisted upon is 60 votes in the United States Senate, no longer can one party alone hold its members and make progress on important issues for the American people. That's why the president has asked again [for] the Democrats and Republicans work together to make that progress."
There were indications earlier in the week that Brown wanted to take his seat early so he could take part in upcoming votes on several Obama administration nominees.
"While Senator-elect Brown had tentatively planned to be sworn into office on Feb. 11, he has been advised that there are a number of votes scheduled prior to that date. For that reason, he wants certification to occur immediately," Brown's attorney, Daniel Winslow, wrote to the Massachusetts governor Wednesday.
But on Thursday, Brown said votes had nothing to do with the quickened schedule.
"I'd rather be here than home," he told reporters as he arrived on Capitol Hill Thursday afternoon.
Brown said he had not spoken recently to Senate Republican leaders. But he said Sen. John McCain, R-Ariz., the 2008 Republican presidential nominee, has become something of a mentor to him.
The most controversial upcoming nominee would be Craig Becker, a lawyer for the Service Employees International Union, to sit on the National Labor Relations Board. He had a confirmation hearing earlier this week and the committee could vote very soon.
Brown did not clamor to be sworn in last week, when Democrats passed, on party lines, a measure to raise the debt ceiling by $1.9 trillion. If he had, Republicans would have been able to unite against the measure.