Megan McArdle wants to know something:

bq. Okay, so Obama’s not the most liberal senator. But who is?

One answer can be found here, in Lewis and Poole’s Optimal Classification ranking of voting patterns the 110th Senate. Here’s a description of the method. This measure isn’t quite “liberal vs conservative” but it does tell you which senators are most alike, as based on their voting records and boiled down to a single dimension. For the Democrats, Russ Feingold, Chris Dodd and Bernie Sanders are on one side, with Tim Johnson, Mary Landrieu and Ben Nelson on the other. In the 110th congress, there are 10 senators who are closer to Feingold than Obama is. (Of course, the 110th session is only half over.) In the 109th, for which there’s complete voting data, there were 20. In the 109th session only three places separated Obama and Clinton — they were ranked 21st and 25th respectively. So far in the 110th session, eight places separate them. It’s Obama who has moved.