Senator Olympia J. Snowe announced her retirement from the Senate Thursday citing growing partisanship in Washington as a reason for her departure. As Paul Kane and Chris Cillizza reported:

Sen. Olympia J. Snowe, the iconic Republican moderate from Maine, announced her retirement from the Senate on Tuesday, saying she would not seek a fourth term because political partisanship has made the Senate unproductive.

“Unfortunately, I do not realistically expect the partisanship of recent years in the Senate to change over the short term,” Snowe said. “So at this stage of my tenure in public service, I have concluded that I am not prepared to commit myself to an additional six years in the Senate.”

Snowe has made a reputation, over 33 years in Congress, as someone eager to build political bridges between moderates from both parties. But in recent years, she has become an increasingly isolated voice in a Congress hobbled by partisan gridlock.

Given her continued popularity, her retirement is a rebuke to the partisanship that has come to define the political times in Washington. Snowe’s willingness to compromise and work with the other side has frequently put her out of step with her party, and Democrats have often looked to her as one of the Republicans willing to break ranks with the GOP leadership. Snowe’s reputation as a moderate has grown as fewer and fewer senators on either side of the aisle have been willing to take bipartisan actions.

But it has also become harder to get elected by advocating moderation and compromise. In recent years, Republican senators in particular have been under pressure from tea party elements in their party to abandon the middle or face primary challenges from the right.

In a sign of the increasingly unique place Snowe holds in the Senate, President Obama issued a statement Tuesday evening praising his former Senate colleague. “Senator Snowe’s career demonstrates how much can be accomplished when leaders from both parties come together to do the right thing for the American people,” he said.

Snowe’s rebuke of partisanship in Washington at a time when her personal popularity is high in Maine should put both parties on notice. As Ezra Klein reported:

According to the Voteview ideological ranking system, the most moderate Democratic senator in the 112th Congress — that’s this session, for those keeping track — is Nebraska’s Ben Nelson. The most moderate Republican senator is Maine’s Olympia Snowe. And as of today, they’re both retiring.

That shouldn’t be a surprise. For years, our increasingly polarized political system has been culling moderate members. To see this in video form, click the clip below. It’s a visualization of congressional polarization beginning with the very first Congress and running right through today. And if you fast forward toward the end, where we are, you’ll see the pattern progress right in front of you: The Ds and the Rs in the middle — those are the centrists — keep disappearing, and the masses of Ds and Rs keep moving farther away from each other.

That’s American politics today: two parties, no touching. And that’s part of why Snowe is retiring. “[W]hat I have had to consider is how productive an additional term would be,” Snowe said in a statement. “Unfortunately, I do not realistically expect the partisanship of recent years in the Senate to change over the short term.”

She’s right. The partisanship of recent years is not likely to change over the short term. In fact, it’s likely to get worse. Snowe will probably be replaced by a liberal Democrat. Nelson will probably hand his seat to a conservative Republican. The parties will become that much more unified, disciplined and polarized. And that’s okay. Or, at least, it would be okay if America’s political institutions were prepared for it.

We use “polarization” as an epithet. It’s what’s wrong with America’s politics. It’s what’s wrong with America’s political parties. It’s what’s wrong with America’s politicians. It’s what’s wrong, finally, with America.

Snowe’s retirement puts the GOP in a tough position in Maine, and the Republican Party will struggle to keep control of her seat in November. As Chris Cillizza reported:

Snowe’s retirement represents a major setback for the GOP’s efforts to regain a majority in the Senate. As a moderate Republican, she may be the party’s only hope to hold a seat in the strongly blue state.

Republicans did get some traction in the state in 2010, including electing Republican Paul LePage as governor.

But in a more neutral political environment, and in a federal race, Democrats will be heavy favorites to steal this seat from Republicans — their best pickup opportunity in the country, for sure.

Snowe’s announcement took Republican leaders completely by surprise; she informed Senate Minority Leader Mitch McConnell (Ky.) and National Republican Senatorial Committee Chairman John Cornyn (Texas) of her decision today, according to a person familiar with the decision.

The person noted that Snowe had given no indication that she was doing anything but planning to run an aggressive reelection bid; she had hired Justin Brasell, a renowned Republican operative, to manage her campaign.

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