WASHINGTON -- People say they don't like partisan gridlock in Washington. But they're voting in ways almost certain to increase it, by punishing politicians who cooperate with the opposing party and rewarding ideological purity that pushes both sides to the fringes.

In the past few weeks, Democratic voters have ousted one of Congress' best-known centrists, Sen. Arlen Specter of Pennsylvania, and forced another, Sen. Blanche Lincoln of Arkansas, into a difficult primary runoff June 8.

Republican activists ended the career of conservative three-term Sen. Bob Bennett of Utah in part because he had worked with a Democrat on a health care bill, which went nowhere. In Arizona, they have sent a once-famous maverick, Sen. John McCain, scurrying to the right to save his political skin.

Aside from good political theater, these dynamics profoundly affect government policy. Efforts to revamp energy, immigration and other big policies have flagged largely because of partisan divisions, especially in the Senate, where filibuster rules allow a united minority party to stop all but a few bills.

Even before these recent primaries, Congress was pushing the limits of partisanship. This year's landmark health care law passed without a single Republican vote. President Barack Obama's visit last Tuesday with Republican senators dissolved into testy exchanges, no thaw in hostilities apparent.

Indeed, to many political activists, merely talking with the other party is unforgivable.

South Carolina's Charleston County Republican Party condemned Sen. Lindsey Graham, R-S.C., last year for undermining "Republican leadership and party solidarity" by working with Democrats on an energy bill that remains stalled.

The American Conservative Union gives Graham a lifetime score of 89.68 percent, a fraction higher than those of GOP stalwarts Orrin Hatch of Utah and Mitch McConnell of Kentucky, the Senate's Republican leader.

In upstate New York, conservative activists were so incensed by a Republican House nominee's centrist views that they backed a tea party candidate and handed a special election to an underdog Democrat. Tea party leaders said the sacrifice was worthwhile.

Voters who want bipartisanship are mostly in the political middle, said Nebraska Democrat Ben Nelson, one of the Senate's most centrist members. Activists on the left and right often dominate the nomination process, and demand ideological loyalty, he said.

"You squeeze the middle out, and then there will be more criticism of the lack of bipartisanship" without an awareness of "why there is less bipartisanship," Nelson said. He noted that a liberal group ran ads attacking him last year when he refused to support a government-run health insurance option.

"It's counterproductive thinking," he said, "and counterproductive voting."

On the surface, at least, people say they want bipartisan or nonpartisan cooperation.

In a January poll by NBC News and the Wall Street Journal, 93 percent agreed there is too much partisan fighting between Democrats and Republicans. In a March Associated Press-GfK poll, 84 percent said it was important that any health care plan have support from both parties in Congress.

Voters' behavior, however, often works against such sentiments.

"People will tell you they don't like partisanship, but their solution is, 'The other side should give in to us,"' said Emory University political scientist Alan Abramowitz, author of "Voice of the People: Elections and Voting in the United States."

Centrist candidates are especially threatened when a comparative few motivated activists choose party nominees before general election voters come into play.

Utah GOP convention-goers who ousted Bennett on May 8 not only derided his efforts to craft a health care bill with Sen. Ron Wyden, D-Ore., but they also taunted him with chants of "TARP, TARP."

His sin? He was among 74 senators who voted for the 2008 bank bailout bill (or Troubled Asset Relief Program) pushed by President George W. Bush, a Republican.

McCain, once a champion of balanced immigration overhaul that rejected massive fence building on the Mexican border, now calls for a "security-first" approach that would "complete the danged fence" and send 6,000 more National Guard troops to the region. The change occurred after hard-core conservative J.D. Hayworth announced his bid to deny McCain the Republican nomination this year.

McCain personifies the ways in which intraparty fights in home states resonate loudly in Washington.

A Senate filibuster in January killed a proposed commission that would have had broad powers to recommend ways to reduce the federal debt. Some senators from both parties opposed it. But particularly damaging was that it won not a single vote from the six Republicans, including McCain, who previously had endorsed the plan.

Many conservatives strongly oppose such a commission because they believe it would lead to higher taxes.

With partisanship surging, Abramowitz sees two possible routes for Congress. One involves continued gridlock and all the public anger and frustration it generates.

The other is a revived effort to change the Senate's filibuster rules, a daunting task that would make it easier for the majority party to enact bills despite unanimous minority opposition, as the House often does.

Leaders of both parties say Republicans probably will gain House and Senate seats this fall, narrowing, if not wiping out, the Democrats' advantage.

"That can only lead to more polarization," Abramowitz said, "and more pressure to change filibuster rules."

The pressure may grow, but a closer Democratic-Republican divide in the Senate will make a rules change even harder to achieve.