Sen. Russ Feingold (D-WI) wrote a good article for Salon about Bush's expected pardons:

An unpardonable use of power

If President Bush cares about his place in history, he should think twice before issuing pardons that call his judgment, and the integrity of the rule of law, into question.

Feingold acknowledges the problem - Bush's pardon power is broad:

The power of the pardon is close to absolute.  Short of interfering with their own impeachment, presidents can pardon whomever they choose. 

I'm not sure why Feingold limits the impeachment exception to the President himself because the Constitution gives President power to pardon "except in cases of impeachment," which means all impeachments - not just the President, but anyone in his Administration who might be impeached (Cheney, Libby, Addington, Gonzales, etc.) 

That aside, Feingold explains why Bush's pardons would be so controversial:

At the end of his term, however, this president should think twice before issuing pardons that call his judgment, and the integrity of the rule of law, into question. 

If President Bush were to pardon key individuals involved in the misdeeds of his administration, from warrantless wiretapping to torture to the firing of U.S. attorneys for political reasons, the courts would be unable to address criminality, or pass judgment on the legality of some of the president's worst abuses.  Issuing such pardons now would be particularly egregious, since voters just issued such a strong condemnation of the Bush administration at the ballot box. 

But what can we do to stop Bush's pardons? 

There is nothing to prevent President Bush from using the pardon in such a short-sighted and self-serving manner -- except, perhaps, public pressure that may itself be a window on the judgment of history.  Everyone who can exert that pressure, from members of Congress to the press and the public, should express their views on whether it would be appropriate for President Bush to use his pardon power in this way.

In other words, we should scream our heads off. That's fine - and we've been doing exactly that. (Our anti-pardon petition has over 45,000 signatures.)

But Congress can do more than scream. Congress can pass resolutions urging Bush not to issue corrupt pardons. And if Bush ignores those resolutions, Congress can impeach him - either before he leaves office or afterwards.

Feingold and other Democrats have avoided impeachment since 2006 to win elections. But now that those elections have been won, it's time to put impeachment back on the table, to make sure Bush's final act is not a gigantic f*** you to the Constitution, the Rule of Law, and the American people.