The Economist has an interesting post with an intriguing slideshow presentation on the evolving relationships in the US Senate. The graph, created by Andrew Odewahn, an information-design expert, maps the voting relationships in the US Senate since 1991 and finds an increasingly polarized US Senate.

Back in 1991-93, there were a surprising number of senators who constituted linked nodes between the main clouds of Republicans and Democrats. Howell Heflin, Richard Shelby, Bob Packwood, William Cohen, Mark Hatfield and Arlen Specter all had significant links across party lines, and it was still possible for Jesse Helms to be off in his own far-right world. Nowadays, it's impossible to get far to the right of mainstream Republicans, because half of the Republicans are already there. By the 104th Congress, following the "Republican revolution" of 1994, the middle space was almost entirely unoccupied, and through the next three congresses only Jim Jeffords, Lincoln Chafee and Arlen Specter dared brave the chasm between the parties. In George Bush's first term, following September 11th, bipartisanship again became an option, and cross-party links proliferated. But by the current Congress, things had gone back to the bad old days of the mid-90s: every single connection between the main clouds of the two parties now runs through Ben Nelson, Olympia Snowe or Susan Collins.

If this were a Facebook community, for the vast majority of either party, the legend next to any member of the other party would read: "You have 0 friends in common." What Barack Obama was attempting to do by visiting the Republican issues retreat was to smack himself down in the middle of that divide. The social divide is, of course, most striking because it appears not to correspond to any reasonable ideological divide; as Mr Obama told the Republican senators, the health-care reform that passed the Senate on party lines is an extremely moderate bill, the most conservative and private-sector-friendly version of universal health insurance imaginable. But that doesn't really matter; the clash in the Senate isn't about policy. It's a war between two cliques. It's not clear whether aggressive friendliness can overcome the drive towards social self-segregation. But the Democrats no longer have 60 votes in the Senate, and if the country is going to get anything done in the next three years, the only way forward is to try and make some friends.

A few other observations about the voting relationships in the Senate:

• The Democrats tend toward to be more fractious. They are often divided into at least two camps, sometimes three.

• Until his retirement in 1997, Alabama Senator Howard Heflin was the Democrat most closely aligned with the GOP. Senator Zell Miller of Georgia in the 107th and 108th Congress voted more with his Republican colleagues than with his party. Senator Ben Nelson of Nebraska is now the Democrat with the closest ties to the GOP.

• While the GOP has generally held together better as a group, it too has seen some fissures developing in the 111th Congress. Not surprisingly Senator Olympia Snowe and Senator Susan Collins, both of Maine, are the ones most likely to vote with the Democrats.

• The Republican Senators who have tended to vote more with the Democrats have largely tended to represent states in New England (William Cohen, Jim Jeffords, Lincoln Chaffee) plus Pennsylvania (Arlen Specter) and Oregon (Bob Packwood, Mark Hatfield, Gordon Smith). Apart from Senator Specter who is now a member of the Democratic caucus, none of the others are in the Senate. In short the pool for bipartisanship initiatives is effectively limited to the two Senators from Maine and perhaps the incoming Scott Brown.