Top Senate Democrats immediately said they would refuse to seat Mr. Burris because of the allegations surrounding Mr. Blagojevich, who was arrested Dec. 9. But some legal scholars said that such a move might not stand up in court, if Mr. Burris chose to challenge it.
A prolonged legal fight over Mr. Obama's former Senate seat could complicate the Democrats' agenda in Washington. Without senators seated in Illinois and Minnesota -- where the Senate election is still being contested -- Democrats can count on the support of 57 senators. That means they will have to peel off three Republican lawmakers to defeat any Republican filibuster aimed at blocking legislation.
The argument for blocking the appointment of Mr. Burris is "weak" in light of provisions in the U.S. Constitution, said Abner Greene, a professor of constitutional law at Fordham University.
The Constitution allows each chamber of Congress to "be the judge of elections, returns and qualifications of its own members."
It takes a simple majority to refuse a seat under Senate rules, which would appear to give Democrats the power to block the Burris appointment. The Senate has refused to seat just four members since direct elections were instituted for the chamber in 1913, said Donald Ritchie, a Senate historian.