A left-wing website, Progressive Punch, has ranked all members of the Senate and House according to a rating which gives the member a score of 100% if they have a perfect liberal voting record. The more conservative the member is the closer is their score to zero.
I don't know whether their ranking system is reliable, but one interesting result they show is this: There is not a single Democrat in the House or Senate who scores under 50% (except Ben Nelson who scores 49.4%) and not a single Republican who scores over it. That's a pretty stark division. I wonder if a similar polarization would have been the case twenty five years ago.
It's also interesting to note that four Senate Republicans are far more liberal than the rest of their caucus, although they're still well below the fifty percent mark. See if you can guess who they are before you check out the rankings here.