U.S. Senate Social Graph, 1991 - Present

Each dot represents a U.S. senator

Senators become connected if they vote together 65% or more during a session

Visualization technique reveals affinity clusters within and across the parties over time

102nd Session Daniel Inouye (HI) Quintin Burdick (ND) Al Gore (TN) Arlen Specter (PA) Bob Packwood (OR) William Cohen (ME) Mark Hatfield (OR) Jesse Helms (NC) January 3, 1991, to January 3, 1993 First Gulf war. Dems hold majority. Fairly weak voting blocks with considerable spread within and across parties. Howell Heflin (AL) Richard Shelby (AL) Democrat Republican independent Unable to identify

103rd Session Jim Jeffords Richard Shelby Mark Hatfield Arlen Specter John Chaffee Note how Dems split into 2 distinct but tight blocks. Repubs are more diffuse. January 3, 1993 to January 3, 1995

104th Session 1994 Republican Revolution. Repubs gain majority of both houses for first time since 1950s. No cross-party connections. Both parties form solid blocks. Jim Jeffords (VT) Arlen Specter (PA) Howell Heflin (AL) January 3, 1995 to January 3, 1997

105th Session Jim Jeffords (VT) Arlen Specter (PA) January 3, 1997 to January 3, 1999 House impeaches Clinton. Dem voting block in Senate reemerges.

106th Session Lincoln Chaffee (RI) Jim Jeffords (VT), becomes independent mid-term Clinton impeachment trial early in this session. Note how Dems generally maintain unity, but Repubs. fracture into 2 groups. Session ends with Bush v. Gore. Arlen Specter (PA) John McCain (AZ) January 3, 1999 to January 3, 2001

107th Session January 3, 2001 to January 3, 2003 September 11 attacks. Iraq war resolution. Strong party unity with slight fracture in Dem “ liberal” wing. Movement in the middle, with Liberal Repubs moving towards Dems, and conservative Dems move toward Repubs.. Ben Nelson (NE) John Breaux (LA) Lincoln Chaffee (RI) Zell Miller (GA) Susan Collins (ME) Olympia Snow (ME) Jessee Helms (NC)

108th Session January 3, 2003 to January 3, 2005 Ben Nelson (NE) Norm Coleman (MN) Susan Collins (ME) Olympia Snowe (ME) Arlen Specter (PA) Dems still have slight fracture. Repubs maintain a slight majority with a solid block. However, slight split with Moderate Repubs like Snow, Collins, and Coleman. Zell Miller (GA), switches parties mid-term

109th Session Ben Nelson (NE) Lincoln Chaffee (RI) Olympia Snowe (ME) Susan Collins (ME) Arlen Specter (PA) Barack Obama Hurricane Katrina, Tom Delay investigation, Terri Schaivo, Abramoff scandal. Dems break into 3 groups, with moderates breaking towards the more liberal members. January 3, 2005 to January 3, 2007

110th Session January 3, 2007 to January 3, 2009 Hillary Clinton (NY) Joe Biden Sam Brownback (KS) Arlen Specter (PA) Gordon Smith (OR) Susan Collins (ME) Olympia Snowe (ME) Ben Nelson (NE) Norm Coleman (MN) Conservative Repubs split into identifiable voting block. Split in Dems is reduced. Democrats increased their congressional majorities at mid-term and Senator Barack Obama was elected President.

111th Session January 3, 2007 to present With Specter party switch, Dems have a 60 vote Majority, as well as continued support from moderate Republicans. However, there appears to be a large split in the core Dem blocks. Looks as though a new group of Repubs could be splitting off from the core. Ben Nelson (NE) Olympia Snowe (ME) Susan Collins (ME) Arlen Specter (PA) George Voinovich (OH) Judd Gregg (NH)