I wrote earlier about the electoral implications of the defeat of longtime Senator Richard G. Lugar of Indiana in the Republican primary on Tuesday. It should make the seat much more competitive and will increase Democrats’ odds of retaining the Senate, although the nominee that Republicans chose instead, Richard E. Mourdock, the state treasurer, is perhaps a very slight favorite over the Democratic nominee Joe Donnelly.
The bigger story here, however, is that Mr. Lugar is the latest in a long line of relatively moderate Republican senators to meet an electoral demise. In fact, most moderate Republicans who served in the Senate just a few years ago will no longer be in the Congress when it meets again 2013. This is quite simple to illustrate.
I took the 55 Republican senators that served in the 109th Congress from 2005 through 2007 and divided them into two groups, moderates and conservatives, according to their voting records as analyzed by the statistical system DW-Nominate. Because there were an odd number of Republican senators in that year, I could not divide them exactly evenly, but I put 27 in the moderate group and 28 in the conservative group, with the dividing line falling between Senator John McCain of Arizona and Senator John Thune of South Dakota.
Of the 27 moderates, at most six will return to the Congress in 2013: Mr. McCain, Susan Collins of Maine, Lisa Murkowski of Alaska, Thad Cochran of Mississippi, Lamar Alexander of Tennessee and Charles E. Grassley of Iowa.
The other 21 have been ousted from the Congress in some way. Eight of them retired or have announced plans to do so. One, Craig Thomas of Wyoming, died in office. Another, Arlen Specter of Pennsylvania, switched parties, then lost the primary election as a Democrat. Nine of them lost the general election as Republicans. And two were defeated in a Republican primary contest, not counting Ms. Murkowski, who lost hers in 2010 but won re-election anyway as a write-in candidate. In total, that’s an attrition rate of 78 percent.
By contrast, just 11 of the 28 conservatives, or 39 percent, have been ousted from office or announced plans to retire, and some (like Larry Craig of Idaho and John Ensign of Nevada) did so because of personal rather than electoral circumstances.
Republicans have added a couple of moderates to their ranks in cases where they won seats on Democratic turf, like Scott Brown of Massachusetts and Mark Steven Kirk of Illinois.
Overall, however, they have very little of a moderate coalition left, and the Republican Senate is starting to grow as conservative as the Republican House.