When he's been convicted.
I've seen some people implicitly or explicitly condemning those Alaskans who voted for Sen. Stevens, and apparently gave him a narrow victory. How could they vote for someone who was pretty obviously a crook? (Set aside those who might think the conviction was unfounded; presumably many voted for him even though they had little reason to doubt the accuracy of the jury's finding.)
Seems to me that it's pretty easy: It seems nearly certain that Stevens will be expelled, which means he will be replaced, likely -- in heavily Republican Alaska -- by a Republican in the next special election. (The question whether there'll be a temporary appointed replacement, also a Republican, is irrelevant here.) And this prospect was clear at the time of the election as well.
So the choice isn't between getting a Republican crook and getting a Democratic noncrook. It's between getting a Republican crook for a very short time followed by a Republican noncrook and getting a Democratic noncrook. Anyone who generally thinks the Republican Party is better than the Democratic Party (e.g., who wanted Republican control in the Senate, or a Republican minority capable of mounting filibusters, or just as many Republican votes as possible) could thus quite reasonably vote for Sen. Stevens, even if he thought Stevens was a crook who doesn't deserve to be in the Senate. The same is true of anyone who supports government that's as split as possible, given his anticipation of a Democratic victory in many places.
This is not exactly the point I made in my "vote the party, not the candidate" post; recall that there was a "truly awful candidate" exception to that rule, which might well apply to Sen. Stevens if Sen. Stevens was likely to retain the office. Rather, my point is that voting for a crook who'll likely be thrown out right away, and replaced by a noncrook of your own party, is much better than voting for a noncrook of the opposite party (or not voting at all, which may also help the opposite party get elected).