The Senate must seat Roland Burris.

Says Walter Dellinger, who was a Supreme Court law clerk (for Hugo Black) when the Supreme Court decided the Adam Clayton Powell Jr. case (Powell v. McCormack):

In Justice Black’s view, one of the worst abuses of power in England resulted from parliamentary majorities wrongly refusing to seat dissident legislators. That experience makes me very wary about the Senate’s barring a person from taking a seat unless its authority to do so is clear. Here it is not.

Dellinger notes that Blagojevich is the governor, he has the appointment power, and he has exercised it:

The charges that he sought bribes to appoint certain candidates to the Senate do not automatically render illegal other official acts of his office like signing laws or pardoning criminals. And because there is no evidence that a bribe was solicited from, or proffered by, Mr. Burris, his appointment is presumptively lawful.

Nor do the other arguments against Mr. Burris’s appointment hold up. The contention by the Democratic leadership that Mr. Burris can be denied a seat because the Illinois secretary of state refuses to sign his appointment papers is without merit — it would confer upon secretaries of state absolute veto power over governors’ appointments.

The idea of delaying and giving the Illinois legislature a chance to impeach Blagojevich makes no sense, because Burris has been validly appointed, so he's in until the term is up in 2010.

There is still a separate question whether Burris can get into court (as I noted last Thursday). Dellinger says:

The Supreme Court decision in the Powell case did leave open the possibility that a Congressional decision finding that a member was not properly elected — in this case, appointed — might be a “political question” immune from judicial review.

But that some reasons for denying Mr. Burris this seat might not be subject to review by the courts means that the Senate should take more care, not less.

Exactly. When the court finds a case nonjusticiable under the political question doctrine, it is because it reads the Constitution as committing a particular constitutional question to one of the political branches of government. It is decidedly different from rejecting a Constitution-based claim on the merits. It is saying that some other branch of government is the final authority on the meaning of a part of the Constitution, and that indeed means that the nonjudicial branch that has this responsibility must look at the Constitution and figure out what it means and then follow it.

It absolutely does not mean that the other branch can simply carry out its political will.... something you'd never guess from watching Harry Reid.