Apart from Biden, at least two lawmakers won't be there for long. Rep. Hilda Solis, D-Calif., was nominated as secretary of labor in the Obama administration, and Sen. Hillary Rodham Clinton is departing Congress to become Secretary of State.

For the lawmakers just settling in, and the veterans returning, the work was already well under way. Aides to Obama and senior congressional officials have been at work for weeks crafting an economic stimulus plan that could reach $1 trillion or more. Tax cuts, federal spending, aid to the states, all designed to inject cash into an economy where consumer spending has plummeted, manufacturing withered and job losses grown.

Obama spent much of the day Monday in Congress, meeting with leaders in both parties and pledging to consider ideas advanced by Republicans.

They, in turn, said they believed his pledge of bipartisanship.

Amid the ceremony, Burris was at the center of a remarkable piece of political theater.

Informed in advance he would not be seated, he traveled to the Capitol, met inside with Senate Secretary Nancy Erickson, then emerged in a downpour to tell reporters, "My credentials were not in order, I will not be accepted, I will not be seated."

He pledged a lawsuit to win the seat.

Said Reid: "If Mr. Burris takes possession of valid credentials, the United States Senate will proceed in a manner that is respectful to Mr. Burris while ensuring that there is no cloud of doubt over the appointment to fill this seat."

One other Senate seat was in limbo as the new Congress convened.

Democrat Al Franken holds a 225-vote lead over former Sen. Norm Coleman in Minnesota, a result certified on Monday by the state Canvassing Board. He has not yet received a certificate of election, and with Republicans threatening to protest, Democrats made no attempt to seat him.

In a bit of housekeeping, Democrats also were repealing the six-year term limit for committee chairman. It was a legacy of the Republican Revolution that swept through Congress in 1994, and in erasing it, Democrats evinced confidence in the strength of their majority status.