Changing Congress
The speculated causes, according to congressional scholar Norman Ornstein: congressional redistricting, the permanent campaign style, the change toward more politically partisan media. Ornstein believes (as do I) that the former is the most important in killing off centrism. But the drawing of partisan House district lines doesn't explain the equally dramatic decline among centrists in the Senate. Here, it seems to me, we need to consider a few other factors:
1.) The increasing inability of Democrats to compete in Senate elections in predominantly Republican states. In the past, such senators (for electoral necessity, if for no other reason) tended toward centrism. In the 2004 presidential election, Bush took 60% of the vote or more in 14 states: Utah (71%), Wyoming (69%), Idaho (68%), Nebraska (66%), Oklahoma (66%), Alabama (63%), North Dakota (63%), Alaska (62%), Kansas (62%), Texas (61%), and South Dakota, Kentucky, Indiana, and Mississippi (each with 60%). A quarter-century ago, in 1980, these states sent 14 Democrats to the Senate--ID: Frank Church; NB: James Exon and Ed Zorinsky; OK: David Boren; AB: Howell Heflin and Don Stewart; ND: Quentin Burdick; Alaska: Mike Gravel; TX: Lloyd Bentsen; SD: George McGovern; KY: Dee Huddleston and Wendell Ford; IN: Birch Bayh; MS: John Stennis. (And even Utah and Wyoming were only four years out each from having had their last Democrat in the Senate--Ted Moss and Gale McGee, both of whom were moderates.) In 2005, these same 14 states sent 5 Democrats to the Senate--NB: Ben Nelson; ND: Kent Conrad and Byron Dorgan; SD: Tim Johnson; IN: Evan Bayh.
2.) The increasing unwillingness of Republican electorates (outside of Rhode Island) to nominate moderate or liberal Republicans. In 1972, the Senate GOP caucus included such moderate or liberal members as Ed Brooke (MA), George Aiken and Robert Stafford (VT), Lowell Weicker (CT), Jacob Javits (NY), Clifford Case (NJ), Charles Mathias (MD), Charles Percy (IL), and John Sherman Cooper (KY). (With the exception of Stafford and Percy, any of this list would now be the most liberal Senate Republican if they were alive and serving today.) Of this list, Case and Javits were both defeated by conservative primary opponents; Cooper's state party was taken over by conservatives led by Mitch McConnell, and Aiken, Mathias, Percy, Brooke, and Weicker were all replaced by Democrats--the latter three involuntarily.
3.) The enormous financial resources required to run for the Senate. Senate races today are so expensive that except for wealthy self-funders, it's almost impossible to stand for the Senate without appealing to the base: there's no centrist equivalent to moveon.org or the Christian Coalition that will help raise the needed funds for moderate Senate candidates.
That people such as Lindsey Graham, John Warner, Robert Byrd, or Dan Inouye could (accurately) be considered what passes for a centrist in today's Senate suggests that the centrist bloc in the upper chamber is likely to continue to erode.