Democrats said they were confident of their standing under Article I, Section 5 of the Constitution, which says “each House shall be the Judge of the Elections, Returns and Qualifications of its own members.” On rare occasion, the Senate has denied seats to candidates whose election outcome was in doubt or who were caught up in corruption.

Yet constitutional experts question the extent of that authority, particularly in light of a 1969 Supreme Court decision in the case of Adam Clayton Powell Jr. of New York. The court found that the House could not bar Mr. Powell, who had been accused of financial impropriety, if he met the constitutionally determined qualifications for age, citizenship and residency.

“I think the best reading of the text of the Constitution and the Powell case together is that the Senate has to seat Burris,” said Abner S. Greene, the Leonard F. Manning professor of law at Fordham University School of Law.

The turmoil engulfing the Illinois seat added to an air of uncertainty surrounding the Senate, which convenes next week, after Democrats were only a few weeks ago celebrating an expanded majority of at least 58....

One rough parallel to the current situation arose in 1947 in a Senate dispute over whether the white supremacist Theodore G. Bilbo of Mississippi should be seated after accusations of voter suppression and campaign corruption. In that case, the Senate found itself deadlocked, and Mr. Bilbo died before the disagreement could be resolved.

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