On what grounds? They could try and make something up, and the lapdog media will slurp it up, and be judge, jury, and executioner. BUT even if the dims do take control, it will only be by the slimest of margins. I doubt if even RINOS's like Specter or McLame would cross over to vote to impeach Bush.
Besides, I imagine the only grounds they'll attempt are along their all to familiar rant "Bush lied, people died". To attempt an impeachment of a President under this type of claim, while we are still at war, will send a strong and inspirational message to our enemies (aiding them), and alienate some of our allies and neutrals. And also demoralize our troops, which in turn will lead to many more deaths, than we have already suffered.
Of course, a war turned against us, with more US deaths would only go to inspire our enemies, and dishearten our citizens, theoretically strengthening the Dims in '08.
I guess if the Dims should take control, you could probably bet on them trying to impeach Bush and Cheney. I mean what are a few thousand or 10's of thousands of deaths, as long as the Dims get back in office.
"""On what grounds?"""
Impeachment would probably center on one or more of the items listed here:
http://www.washingtonmonthly.com/archives/individual/2005_11/007556.php