(The paper was co-written a by a political scientist at Duke University and one at the University of North Carolina. If those two schools can come together, can't we all?)
What the duo did was analyze the voting records to find similarities. They then grouped those similarities -- Yahoo's Chris Wilson did a similar experiment here -- into a single chart comparing the likelihood of uniform party line voting over time.
Their conclusion? "We have not seen the current level of partisanship since the early 1900s," write James Moody of Duke and Peter Mucha of UNC. Later, they add: "In the current era, middle positions seem fragile and even longtime middle residents follow party lines (Jim Jeffords) or lose their seats (Lincoln Chafee)."