The announcement this week that both Kent Conrad and Joe Lieberman will not be seeking reelection is a stark reminder of how polarized American politics is today, as well as why it has gotten that way.

Both Conrad and Lieberman would have had difficult paths to reelection in 2012, though for different reasons. Lieberman would have had a hard time winning nomination in the Democratic Party, meaning that to win reelection he would have to run as an indpendent once again. In 2006 he was able to do that because Republicans bailed on their party's nominee and decided instead to back him. A more credibile GOP challenger -- like Linda McMahon -- could certainly win Connecticut in a three-way contest, so Lieberman also would've had a real challenge on his hands from the Republican Party. Ultimately, Lieberman's political troubles stem from the fact that he has been too conservative on foreign policy issues for his liberal electorate in the Nutmeg State, and since the Bush administration, this is deal-breaker.

Conrad, on the other hand, is from a relatively conservative state that has historically been a reliable supporter of the Republican Party. A Democrat wins a state like North Dakota by cultivating a significant amount of cross-over appeal. Indeed, in 2006 Conrad was reelected with 40 percent of the vote from self-identified Republicans. But those days appear to be gone. Earl Pomeroy -- the state's lone House member until this last election -- had historically been reelected via a similar path, but he lost reelection in November by 10 points, which is a very large amount for an incumbent who is not bogged down by a scandal. Pomeroy's decisive defeat was a harbinger for Conrad, who, like Pomeroy, had voted in favor of Obamacare. Worse for Conrad, he is the chairman of the Senate Budget Committee, and thus would inevitably face a unique level of political heat for the budget deficit. Ultimately, Conrad's troubles stem from the fact that he has been too liberal on domestic policy issues for his relatively conservative state, and since the start of the Obama administration, this is a deal-breaker.

What we have, then, are two senators who would face political trouble because they are out-of-step with their electorates on two entirely different sets of issues. If you want to appreciate why the country is so polarized, that's it in a nutshell. During the Bush administration, we were deeply divided over foreign policy. We still are, in most respects. Now, during the Obama administration, we are deeply divided over domestic policy.

(This is also why I think any kind of debate on the national "dialogue" is such a red-herring. The rhetoric in this country is as harsh as it is because we are divided over real things. Our choices on these matters will fundamentally affect the direction in which our country heads for perhaps a generation. No wonder the disource is so vitriolic!)

My guess is that Conrad and Lieberman will not be the only senators of the 2012 class to retire in the face of daunting reelection odds. Look for Ben Nelson to see the writing on the wall sooner or later, and it remains an open question whether Jim Webb wants to go back to the Old Dominion to defend his vote on Obamacare. As for the long-term trend in Congress, I think these retirements are an early indication that 2012 is going to be another election -- the fourth straight, actually -- where we see an increased level of ideological sorting: conservative Republicans representing deep red states or districts and liberal Democrats representing deep blue states or districts. So long as the country is so divided over such fundamental issues, we'll remain on that trajectory.