Here's a very neat interactive tool from Yahoo that lets you map the similarities of different senators. This is an image of how the senate looks when you ask which senators vote with one another 75 percent of the time:
Basically that's your familiar polarization story. The Democrats all vote together, the Republicans all vote together, Susan Collins is in the middle, and Lisa Murkowski is eccentrically similar to Collins but not to any other legislator.
But beef the threshold up to 85 percent and something different happens:
A few Democrats break from the pack, but the Republican bloc totally melts down. And that's despite the fact that in the real world the GOP is more ideologically unified than the Democratic Party. What you're seeing, I think, is the power of agenda control. Because Democrats have the majority in the Senate, Democrats have a disproportionate level of control over the calendar. They use that control to try to ensure that the Senate takes votes on subjects that unify Democrats and avoids issues that divide them.