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.
Interesting, although I don't love Volokh's proven willingness to give in to the two party system.