I'll let Erza explain:
Andrew Odewahn has put together a fantastic set of slides showing the slow deterioration in Senate bipartisanship between 1991 and 2009.
But keep an eye not just on the deterioration of cross-party cooperation, but the shifting blocs within both parties. As Odewahn shows, "there are clear and distinct moderate blocks within both parties." Saying that Democrats have 60 votes doesn't tell you whether Harry Reid has 60 votes, or whether liberals have 60 votes. The story of the Senate is not just that the two major parties don't agree and don't cooperate with each other, but that they don't always agree and cooperate with themselves.
U.S. Senate Social Graph, 1991 - Present
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