By the company he keeps
A funny from Amy Sullivan:
One of the complaints I often hear from conservatives is that religious liberals are moral relativists, that they don't believe in evil. To which I respond that my liberal and religious mother believes very much that evil exists and that it is personified by Dick Cheney.
That usually gets a big laugh out of everyone else and a shamefaced chuckle out of the conservative, who is forced to admit that sometimes evil really is in the eye of the beholder.
Good joke. And, as Amy points out, it even gets chuckles from conservatives.
Many of us in the Bush-Haters-Of-America group find it hard to resist the urge to bash Bush whenever the talk turns to politics in mixed company (mixed meaning Democrats and Republicans). The problem with that is that, despite Bush's obvious failings, there is still a large reserve of good will towards the guy, even from people who disagree with his policies. If you start talking about Bush being evil or just simply the worst president ever, you will quickly lose the argument with these people.
But, if you bash Dick Cheney, Donald Rumsfeld or John Ashcroft ... well then that's not quite so bad. You might even get the aforesaid chuckle of agreement.
Thus I would suggest an alternate strategy for bashing Bush: don't bash him personally. Just direct his share of the anger and frustration you feel onto those around him (excepting Powell. He also has a reservoir of good will that can be dangerous to attack directly.) Many people may be frustrated with Bush, but don't feel comfortable bashing him directly. But they might be agreeable to using a Bush substitute like Cheney, Rumsfeld, Ashcroft or Rice (most people probably don't know who Wolfowitz, Pearle, etc. are, so don't bother with them).
Then, when they are really into the whole "bash the Bush crowd" celebration, point out to them that re-electing Bush means four more years of those guys as well.