Last week, I explained how a bunch of moderate and liberal Republicans getting elected is consistent with a larger story—my expectation that the new 112th Congress will be the most polarized yet, even more so than the record-setting 111th.

How will that happen? On this election day, I’ll detail one path that has received relatively little attention.

While everyone focuses on the Republicans’ pickup opportunities—of which there are many in the Senate and the House—fewer observers have taken a cumulative look at the consequences of open seats won by the incumbent party. That is, when a Republican replaces a retiring (or defeated) Republican, and similarly for Democrats. Since not all partisans are alike ideologically, it behooves us to examine more closely who is replacing whom.

I will address Senators today, and only Republicans. I will focus in this post on the latter for a very simple reason. Because of the coming Republican wave, only three Democratic open seats exist which other Democrats have a (virtual) lock on. These are Connecticut, Delaware, and West Virginia. The former are both liberal states, but I have no prior legislative voting behavior to predict the likely ideologies of Richard Blumenthal and Chris Coons, respectively, with respect to the incumbents Chris Dodd and Joseph Biden/Ted Kaufman. Nor can I tell if WV Governor Joe Manchin will be more or less liberal than Carte Goodwin, Robert Byrd’s replacement by Manchin himself.

So are the new Republican Senators going to be more conservative in the 112th Congress? Based on evidence culled from my research, and a little speculation, I will say yes with a large degree of confidence.

To proceed, I will divide Republican open seats going Republican (ROSGR) into five categories, based on the state of evidence that the seat is becoming more conservative, or not.

In the first category are those ROSGRs that I have strong evidence are becoming more conservative. This is because of the well-known finding in political science by Keith Poole and Howard Rosenthal that politicians “die in their ideological boots” (see here for an ungated version of Keith’s paper). That is, politicians rarely change their stripes. Thus, we can look at their prior voting records in other legislative chambers—federal and state—to predict their Senatorial ideology.

  1. Florida. Incumbent Mel Martinez retired and was replaced by George LeMieux (via a pick by Governor Crist). Both were moderately conservative; about three quarters of congressional Republicans were to their right. Marco Rubio, former speaker of the Florida House of Representatives and victor against Crist in the Republican primary, is a few clicks to their right (but less than I’d expected).
  2. Missouri. Retiring Kit Bond is a moderate; he’s about as conservative as LeMieux or former Senators Spencer Abraham or Pete Domenici. Roy Blunt is the current Representative and minority whip from the 7th Congressional District in Missouri. He’s also the likely victor on Tuesday. He’s compiled a voting record that’s 20 percentiles more conservative than Bond, about in the range of Mike Enzi of Wyoming.
  3. Ohio. Retiring George Voinovich was one of the most liberal Republicans in the Senate. Representative Rob Portman (OH-2) , his nearly-guaranteed successor, is 20 percentiles more conservative. His ideology is about the same as that of former Senator and majority leader Bill Frist.

In the second category, we have ROSGRs where I have informed speculation and a strong degree of confidence (but not nearly so much as the first category where I have evidence) that will be more conservative than the previous incumbents. Both are the open seats resulting from the defeat of incumbents in primaries, on grounds that the latter were too liberal.

  1. Utah. Senator Bob Bennett is in the moderate half of congressional Republicans; that’s still pretty conservative by national standards. Still, Utah is one of the most—if not the most—conservative states in the union. It can electorally support a much more conservative Senator. While I have no voting record for Mike Lee, every bit of evidence from the Republican primary tells me that he’s likely to be far more conservative.
  2. Alaska. Assuming Joe Miller wins, that means he’d replace Lisa Murkowski, a decidedly moderate Republican. The primary and general campaigns have clearly staked ideological territory where Miller is to the right of Alaska. As in Utah, Alaska is one of the most conservative states in the US; it can easily support a more conservative Senator.

In the third category, we have a ROSGR where I’m making an informed guess with less confidence than the previous category. There’s only one of these:

  1. Kentucky. Jim Bunning is a pretty conservative Republican; only about a fifth of congressional Republicans are more to the right. That causes me to move this race out of the last category. On the other hand, Republican nominee Rand Paul is running a distinctly ideologically conservative and libertarian campaign. This leads me to believe he’d be more conservative than Bunning, but I’m not as sure.

In the fourth category, we have a ROSGR where I have evidence that there won’t be much change in ideology after the incumbent leaves. There’s only one of these.

  1. Kansas. Despite his reputation, Sam Brownback is about in the middle of his party for conservatism, and is retiring to run for governor. Representative Jerry Moran (KS-1) is a lock to win Brownback’s seat, and he’s just about as conservative.

In the last category, we have the Republican counterpart to Delaware and Connecticut: a race where I have no evidence nor good speculation about how the winner will compare to the retiring incumbent.

  1. New Hampshire. Judd Gregg is a moderate Republican, but that’s probably about the carrying capacity of the state for conservatism. Kelly Ayotte is the former state attorney general, and not much can be gleaned from the campaign (apart from her defeat of more Tea Party-favored Ovide Lamontagne).

So, to sum up, of the eight open seats being vacated by Republicans and likely taken again by Republicans, 5 will become more conservative with a high degree of confidence, 1 will become more conservative with a medium degree of confidence, 1 will stay the same, and 1 is unclear.

Moreover, I didn’t include Pennsylvania’s Arlen Specter, who quite clearly would have been an example of a ROSGR had he remained a Republican. He was the most liberal Republican in the Senate at the beginning of the 111th Congress–more liberal than either Susan Collins or Olympia Snowe. Had he stayed a Republican, he would most likely have been defeated in a primary by Representative Pat Toomey (PA-15), which would have set up the same general election we are seeing today. Likely victor Toomey is very conservative; only 20 percent of Republicans are more to the right. In fact, I see him as more conservative even than Rick Santorum. So, depending on how you look at this case, the case for ROSGRs going more conservative is even stronger.

All of this is one reason why the 112th Republicans will be the most conservative, and the 112th Congress will be the most polarized yet. This is the “dog-bites-man” story; while the “man-bites-dog” story of the coming liberal House Republicans is merely an entertaining sideshow.