Daniel Sunshine, director of strategic content for our friends at National Journal, put together this fascinating chart as part of a presentation that is one of the benefits of the National Journal Membership program. It's based on over 30 years of data from National Journal's Congressional vote ratings (the 2011 ratings can be found here). As you can see something remarkable happened last year: any remaining ideological overlap between the Democratic and Republican parties totally disappeared in the Senate, as the vote ratings, for the first time, were divided neatly by party line.  This goes a long way to explaining why Congress failed to do much of anything last year: the Senate -- with its reliance on supermajority procedural votes -- was not designed to be does not operate as a partisan, majority rules body like the House of Representatives. [Ed. noteAs The Atlantic's James Fallows gently reminds us, we are guilty of  the "false equivalence" he has long documented in the political press by using the word designed when we talked about the Senate's procedures. We should have said that it is a majority rules body that has only in recent decades been turned into a supermajority gridlock machine. Mea culpa.]

Over the last three decades, liberals have been banished from the G.O.P., while Democrats who vote regularly with conservative Republicans have also disappeared. On the edges, sometimes that's happened by switching sides (on the 1994 chart, the most conservative Democrat, Sen. Richard Shelby, switched parties the next year, while Sen. James Jeffords, the most liberal Republican, switched parties in 2001.)  But the middle has also been a politically dangerous place to be. The most conservative Democrat highlighted on the chart for 2011, Sen. Ben Nelson of Nebraska, who rated the Senate's 48th most conservative, announced in December that he wouldn't run for re-election. (Both of the Senate's independents, Joe Lieberman or Bernie Sanders, are not included in the chart.) The most liberal Republican, Sen. Susan Collins of Maine, ranked the 47th most liberal member in the chamber, is not up for re-election until 2014. But the senior Senator from her state, Sen. Olympia Snowe (ranking just behind Collins at 46th) last week announced she would not seek re-election this November.