Trying to prosecute key Bush Administration officials on what are viewed as trumped up, politically based charges would create a firestorm of partisanship and outright hatred that would surpass anything in American history since the Civil War. Members of a political party in the United States, whether it be Republican or Democrat, are simply not going to stand by idly with their hands in their pockets while their political views are criminalized.
At best, this would lead to tit-for-tat prosecutions. By that, I mean if Democrats throw George W. Bush in jail for ten years, Republicans will do their best to find an excuse to throw Barack Obama in jail for ten years -- and don't think it can't happen. The American political system tends to be cyclical and so today the Democrats may be on top -- but in four to eight years, when Obama leaves office, it's entirely possible the GOP could be in charge of both Houses of Congress -- and looking for an opportunity to get payback for Bush. Again, that is the best case scenario. The worst case scenario could mean blood in the streets, riots, and a breakdown of the "orderly transfer of power" that has always been a hallmark of American democracy.
One of the reasons that has never been an issue previously is precisely because the loss of power for an American politician doesn't mean that he's threatened with the loss of his life or liberty. If we throw the rule of law out the window and leaving office may mean a prison sentence or worse, those "orderly transfers of power" we have in this country are going to begin to break down -- and politicians will use any means necessary to remain in charge. It's understandable if that sounds farfetched since events of that sort haven't happened on a widespread scale here since the Civil War, but there are many nations across the world where a change of leadership is a terrifying and violent ordeal for the populace. There's very little to be said for potentially joining their ranks.
However, if Krugman, Conyers, Pelosi, Biden and Company get their way and we abandon a practice that has helped assure political stability for more than two centuries in this country, then we should not be surprised if America starts to experience the same troubles as the unstable banana republics that we've always urged to follow our example. That would be a truly disastrous turning point for our democracy and that's why it's highly irresponsible for Democratic officials to even publicly speculate about going down that dark and perilous road.