The surprising retirement of moderate Republican Sen. Olympia Snowe of Maine moves congressional centrists a step closer to extinction and highlights the great paradox of American politics.

Voters say they want bipartisan solutions to the nation’s problems. But they congregate and vote in ways that ensure partisan warfare, driving the GOP farther right and the Democratic Party farther left.

Even with her party standing a good chance to regain the Senate majority, Snowe wanted no more of the endless gridlock that has rendered Congress barely able to carry out the most basic functions, such as keeping the federal government’s doors open.

She expressed frustration “that an atmosphere of polarization and ‘my way or the highway’ ideologies has become pervasive in campaigns and in our governing institutions.” She told MSNBC on Wednesday that “the political paralysis has overtaken the environment,” hurting the country.

Some congressional scholars said Snowe’s retirement is discouraging but not surprising.

“It puts a human face on a sad truth,” said William Galston, a former Clinton White House aide and co-founder of the bipartisan advocacy group No Labels. That truth, he said, is that “especially in Congress, the polarization of our party system has now reached the point where building bridges has become almost impossible when the issue is one of any significance.”

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“On most fundamental issues,” Galston said, “the center has disappeared for all practical purposes.”

Snowe is one of the few remaining moderate Republicans, a group that once dominated the Northeast and vied for control of the national GOP under leaders such as Nelson Rockefeller. She was instrumental in forcing President George W. Bush to limit the size of his 2001 tax cut. She was one of three Senate Republicans who backed President Obama’s 2009 stimulus plan.

But Snowe found it increasingly difficult to reach across party lines that kept moving farther apart. She joined all other Senate Republicans in opposing the final version of Obama’s 2010 health care overhaul. And she grew weary of the constant pressure to bash Democrats on everything and to expect the same in return.

“She just quit in disgust,” even though she easily could have won a fourth term this fall, said Matt Bennett of the centrist-Democratic group Third Way.

“It’s very, very bad for the institution to be losing the dean of Republican moderates, if there are any,” Bennett said.

An annual study by National Journal found that the congressional realignment of the two parties is virtually complete, with the GOP solidly identified with conservatives and the Democratic Party clearly seen as liberal.

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“For the second year in a row,” National Journal reported, “no Senate Democrat compiled a voting record to the right of any Senate Republican, and no Republican came down on the left of any Senate Democrat.”

The House is similarly divided. Only six Republicans “compiled a slightly more ‘liberal’ voting record than the most conservative Democrat, Rep. Dan Boren of Oklahoma,” National Journal found. GOP presidential candidate Ron Paul of Texas was one of the six, “because his libertarianism takes him so far right that on some issues he … pops up on the other side,” the report said.

Reasons for the polarization are well known, even if they are rarely explored in day-to-day conversations and reporting about legislative impasses and voter frustration.

Race relations that followed integration moved the great majority of Southern whites into the Republican Party, while blacks solidified their Democratic loyalties sown by Franklin Roosevelt. As Sun Belt conservatives ascended in the GOP, they drove away Northeastern liberals and moderates.

In state legislatures throughout the country, both parties colluded to redraw U.S. House districts to make them either safely Republican or safely Democratic. With nothing to fear but a loss in their own party’s primary, Democrats drifted farther left and Republicans shifted right, protecting their flanks and widening the gulf in Washington.

Mobile Americans exacerbated this trend by settling among like-minded people. Big cities, the West Coast and the Northeast became increasingly Democratic. The South and Great Plains became increasingly Republican. And industrial states such as Ohio and Pennsylvania, plus retiree haven Florida, became perennial battlegrounds.

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Activists in both parties, meanwhile, took greater control of the nominating process. Some tea partiers in particular vowed to punish anyone who dared compromise with Democrats. Longtime Republican Sen. Bob Bennett of Utah became their first victim, sending a chill throughout the party.

Centrist and independent voters, who paid little attention to primaries, became increasingly frustrated with a partisan-driven Congress unable to reach accord on taxes, spending, Medicare reform and other issues.

“The public wants people to work together, and yet we keep electing people who don’t do that,” said Mickey Edwards, a former Republican House member from Oklahoma who now writes about Congress. The problem, he said, is that congressional candidates “first have to get through a party primary in which relatively small numbers of people are voting. They tend to be more extreme people.”

“Candidates who might be willing to work across the aisle don’t get elected because they can’t get through the primary,” said Edwards, whose forthcoming book is titled “The Parties vs. the People.” As examples, he cited Bennett and Mike Castle, the former House Republican who lost the Delaware Senate primary to tea party favorite Christine O’Donnell, who in turn lost a general election that Castle was favored to win.

By some measures, Snowe is the Senate’s most liberal Republican and Ben Nelson of Nebraska is its most conservative Democrat. Both are retiring this year, raising serious possibilities they will be replaced by less moderate members of the opposite party, further widening the chamber’s partisan divide.

Sen. Lindsey Graham, R-S.C., said Wednesday that ideological differences are fine, provided they don’t block the higher goal of reaching accord on pressing problems. “There’s nothing wrong with being a liberal or conservative,” Graham said. “What’s wrong is we can’t find common ground on issues that require it.”

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Some South Carolina Republican groups have lashed out at Graham for not toeing the partisan GOP line on every issue.

Galston says the political parties are somewhat more polarized than the American people. “That mismatch creates a substantial pool, tens of millions of voters, who feel uncomfortable with the polarized state of affairs,” he said. This group possibly could become “a major political movement” that would force the major parties to respond.

“If it doesn’t,” Galston said, “it’s not clear to me what disrupts this pattern” of growing polarization.

“To some extent, it’s self-perpetuating,” he said. Disgusted centrists turn away from politics and leave decisions to extremists on both sides.

Olympia Snowe, Galston said, “is the poster child” for the frustrations of Americans who want the parties to drop their bickering and settle the nation’s problems. A year from now, she will be out of office.