Charles Pierce subbing for Erica Alterman on Media Matters does a good job of voicing the reasons for impeachment:

For the past couple of weeks, they've just gotten blatant about it. The administration of George W. Bush is bound by no law, bound by no precedent, bound not even by the forms of democratic self-government, let alone its actual substance, which is being used as a throw-rug in John Yoo's den these days. They will torture and the Congress can do nothing. Their powers to spy, to search, and to seize are unlimited and Congress is not remotely entitled to know even what those powers are. They can imprison without trial. They can force corporations -- and, indeed, individuals within the government -- to violate the law. They are not subject to treaties. They are not subject to oversight, nor even subpoenas. Read this swill from yesterday. Through his actions, and from the mouths of his minions, George Bush is now claiming fully the powers of a tyrant, by any reasonable definition of the term.

This is the only issue in the presidential campaign. It is the only truly existential threat to the country. Everything else -- health care, climate change, campaign finance, the deficit -- mean nothing if we fail on this fundamental issue. I don't know where the two Democratic contenders fall on this stuff -- their campaigns have been damnably vague about it -- but I know John McCain will be immeasurably worse. His anti-torture bill allowed torture. His "compromise" on judicial nominations allowed the Democrats to maintain the right to filibuster as long as they promised never to do so. This allowed Roberts and Alito to skunk through in order to deface the constitutional order, likely for the rest of my lifetime, and McCain has promised to let a theocratic loon like Sam Brownback to help him pick his own judges. He's always had a sweet tooth for executive power; his line-item veto was so nakedly unconstitutional that even William Rehnquist noticed. And, yesterday, he got up in front of the CPAC crowd that earlier had cheered every single one of the steps toward tyranny that the administration had undertaken. A while back, MoveOn.org said unkind things about a soldier in a newspaper ad, and the entire capital got the vapors. The Congress of the United States was moved to resolve to condemn the newspaper ad. Democratic politicians rushed to sign on. Now, a group of very obvious extremists -- Dick Cheney is an authoritarian bully and a personal coward. His approval rating is 19 percent in the country and 100 percent in that hall. Res ipse loquitur. -- gathers in Washington, and not only do the party's most prominent political figures truckle and beg, your liberal media puts the worst of them on the air, as if they were serious people and not simple public vandals. Jesus Christ in Air Jordans, what in hell was David Bossie, a thug and a hoodlum, doing on Jim Lehrer's program last night? Tom DeLay is under indictment, for pity's sake. Why was he on MSNBC, grinning at Chris Matthews and lying about climate change? Mitt Romney's speech was a sprawling landfill of demagogic swill. It was treated as, well, statesmanlike by people who believe that John McCain is not conservative enough. This is plainly nuts, and any respectable conservative would work tirelessly to wring these crackpots out of the movement before the whole mess goes over the cliff again. Somebody should, you know, take out an ad or something.