I would further note that looking at what legislation a Senator has or hasn’t cosponsored can get pretty misleading. Cosponsoring a piece of legislation that’s not going anywhere is the ultimate in congressional cheap talk. What’s more, Senators sometimes sign on to legislation they don’t actually favor (see the Republican co-sponsors of Ron Wyden’s bill) in order to be able to change things around. And sometimes taking a position in favor of something actually means the reverse — I recall a noteworthy moment at YearlyKos last summer where Clinton was trying to brush off worries about her commitment to political reform by noting that she’s on record in favor of full public financing of federal campaigns. Well, I favor that, too, but it’s not going to happen and Clinton’s never done anything to try to lay the groundwork for making it happen. In effect, her support for public financing is a beard to give cover to her opposition to smaller-scale, more realistic political reform proposals.
Which isn’t to say that she’s uniquely duplicitous in these regards, I just happen to be familiar with that particular issue. You see something similar with John Dingell and the carbon tax, and, indeed, with tons and tons of other legislators. If you want to understand people’s records, you have two decent options. One is to go totally “dumb” and use something like DW-NOMINATE which soaks up every single vote cast. There the size of the data set helps iron out the flaws in the individual data points and gives you a crude big-picture sense of where things stand (both are liberal, and Obama somewhat more so). The other way to go is to go “smart” in which case you need to actually speak to people who work with congress on some issue you’re concerned with and ask them how helpful or not some Senator or other has been on some issue. That’ll involve voting records, cosponsored legislation, media appearances, floor statements, timing, committee antics, secret promises, friends of friends, etc.
Going for the middle ground seems appealing, but the odds are that it’s just going to wind up misleading.