Democratic officials sketched out several situations that could keep Mr. Burris from becoming a member of the Senate. For example, while the Senate delays admitting him, Mr. Blagojevich would be removed as governor after a vote next week by the Illinois House, and then a trial in the State Senate; Lt. Gov. Pat Quinn would become governor; and Mr. Quinn would appoint a Senate replacement  all before Mr. Burris was allowed to take the seat.

The impeachment committee will invite Mr. Burris to testify before it, Illinois House officials said Friday, though a time and day has yet to be named.

Under the developing strategy in the Senate, Democrats intend to prevent Mr. Burris from making his case on the Senate floor or to steer the appointment to the Rules Committee for an inquiry in the hope that Mr. Blagojevich leaves or is forced from office before any showdown.

To reach that point, Democrats plan to enforce a rule requiring that credentials presented by incoming senators be countersigned by both a state’s governor and secretary of state.

The Illinois secretary of state, Jesse White, has said he will not sign the document, and Mr. Burris is seeking a court order requiring him to do so. That matter is awaiting a ruling in the Illinois Supreme Court, where an answer, legal experts said, seemed unlikely until at least the middle of next week.

If Mr. Burris were able to get his credentials countersigned, he could by Senate custom be allowed to go on to the floor and seek to be sworn in along with the other incoming senators. But at the point that his swearing-in is imminent, Democrats currently plan to raise an objection and seek to have the question of Mr. Burris’s qualifications referred to the Rules Committee for up to 90 days or some defined period.

Mr. Burris’s case is also spilling over into the still-unsettled results of the Minnesota election. With legal proceedings continuing in the race for the seat now held by Norm Coleman, a Republican, officials of both parties say it is unlikely that Democrats would try to seat their candidate, Al Franken, without final paperwork at the same time that they are trying to deny a seat to Mr. Burris on those grounds.