This is a question that's never been adequately answered to my satisfaction by those who oppose impeachment. You all often claim that impeachment will be a distraction, and that if we pursue it, it will cause us to lose the Presidential election. Why? It doesn't make any sense.
First of all, Presidential candidates will have nothing to do with the impeachment process, unless and until it reaches the Senate, and even then, not all of them will have to vote in the trial. Only two of the three frontrunners are serving Senators. Whether someone should be the next President has nothing to do with whether we should impeach this one.
Second of all, how exactly does a parade of Bush and Cheney's crimes and Constitutional abuses hurt Democrats in an election? You claim it would be divisive, who would it divide? The 25-30% dead-enders? We wouldn't win them if we promised to make them all billionaires and actually followed through. Would it piss off independents, who after all, are independents because they don't really have the time or inclination to follow politics closely? They already hate Bush by a two to one margin. Impeachment piles on the reasons to hate him even more. It would be one thing if Bush has done even a single thing that has made the lives of Americans better, but after Iraq, Katrina, a squandered opportunity after 9/11, an increasingly hollowed out economy, the list goes on and on, there is no fall-back of real gains Bush or his allies can point to in order to rally support. If they try, they go down with him, and if they try to ignore the issue or dance around it, they get busted for fiddling while Rome burns.
Like it or not, this next election is all about Bush. Not because he's running again, but because Americans desperately want change. They want something other than Bush and his brand of Republicanism. A winning campaign is one that will point out exactly where and how Bush is bad and that commits to not doing those things and installing policies and laws that prevent those things from ever happening again. Impeachment provides the material to do the first half and showcases the conviction of the party and the candidates that these things really are bad and should be stopped.
On the other hand, the position of refusing to impeach is one which implies that the person who takes it believes there are still some redeeming qualities to Bush and Cheney that should be preserved. Not only is that a poor way for Democrats to draw the distinction between themselves and their opponents, but it's a proposition you'll likely not find much support for in the general populace.