And the answer, of course, is the Democratic Party.
The Washington Post tracks the votes of all representatives and senators, and also tracks how often a particular member votes with or against his party leadership. We can examine these numbers pretty well through these links:
Now, the most partisan member of the House was Republican Charlie Norwood, who voted with the leadership 100% of the time. However, this tally was for nine votes only, after which Norwood died. The second most partisan Republican is Jo-Ann Davis, who also suffers from a vote-skewing problem. She has missed most votes this year because of illness.
Even with this, though, she is only about 174 on the list. Between her and the late Charlie Norwood are a whole slew of Democrats who essentially vote the party line nearly all of the time.
The same thing can be seen on the Senate side. The most partisan Republican is John Barrasso of Wyoming, who only took office June 25th to replace the late Sen. Thomas. Above him are 21 Democrats. And at the bottom of the list, where the least partisan senators are listed, Republicans dominate.
It is clear that however much Democrats say they value bipartisanship, they don’t run their party in a way that encourages it. Debates here prove that - any Democrat that votes for a Republican proposal is savaged. And these numbers bear that out as well.