however implausible it is that a bill of impeachment would be returned against Bush--and I've more or less given up on the prospect--it's possible that if Bush keeps up his craziness, he could trigger it. Certainly if during one of the several imminent Congressional investigations he and/or Cheney acts too highhandedly or are discovered to have engaged in a coverup of some significance, I could see some Democrats calling for impeachment. And with the list of those condemning his handling of the Iraq war growing ever more weighty, I could imagine that if he called up more troops, the American public would demand some way to stop him, and if the threat of impeachment would do it, then perhaps even that.
(BTW, in view of the state of affairs in Iraq, as well as of the military, consider how cynically nuts McCain is, with his call for more troops. He's clearly posturing for the presidential election in 2008, stirring up the Rightwing nutcases at Bush's expense, knowing that if no more troops are sent and Iraq explodes (as it will, no matter what), he can play "I told you so.")