A hypothetical question
Suppose Bush is impeached for the alleged violation of FISA (the wiretapping issue). There are rumblings about rumblings that this would occur (although I doubt it, unless there is a seriou shift in the balance of power in Congress.) ...but let's say it happens.
There is little question that the administration's actions do not comply with FISA. I haven't seen anyone make the argument that they do comply, and I actually heard Karl Rove (in a spech to the Republican party) state pretty clearly that they did not. However, there is a legitimate Constitutional question with regard to whether Bush is subject to FISA. The treatment of prisoners during wartime is an exclusively executive function. Congress can't eliminate executive functions (although, according to the Youngstown decision cited in a previous post, Congress can place limitations on them.)
Were this hypothetical impeachment an ordinary criminal proceeding, the Constitutional issue would be raised in court. However, impeachments are handled in Congress, not a court.
If Bush were impeached and convicted, would he have the opportunity to appeal the Constitutional issue? Does the Supreme Court (or any court) have the ability to overturn an impeachment on Constitutional grounds (i.e., that there was no "high crime or misdemeanor," because the law that was violated was not a Constitutional restriction on executive power)?