In the high plains and Rocky Mountain foothills around Denver, a handful of suburban, moderate voters may control the fate of Congress.

A new, extraordinarily balanced congressional district is up for grabs here, and it has become one of the most important political battlegrounds in the country, as Democrats seek to gain 15 seats and take control of the U.S. House. Colorado is one of a growing number of states where party allegiances are fragile enough to give decisive influence to middle-of-the-road voters.

Here, a backlash against stringent antitax policies and the influence of religious conservatives has hurt Republicans. For the first time in more than 40 years, Democrats in 2004 won control of both houses of the state legislature, and polls this month show Bill Ritter, the relatively conservative Democratic gubernatorial candidate, 17 percentage points ahead in the race to replace outgoing Republican Gov. Bill Owens.

"The state is clearly shifting from red to more purple," said political analyst Eric Sondermann. "Maybe we were the canary in the coal mine for the country - the evolution happened here in 2004." Sondermann said Colorado voters' concerns were similar to those around the country: "fatigue with Iraq, disillusionment with President Bush, and 9/11 getting more remote."

"Things have changed," said John Straayer, longtime political science professor at Colorado State University. "Part of what you're seeing is a broader migration back to moderate politics."

To try to attract those moderate voters, the Republican and Democratic national parties may pour $10 million to $12 million into Colorado's Seventh District, an open seat in a microcosm of the nation.

The district was created in 2002, with boundaries imposed by a state judge in Denver, specifically designed to be that rarest of political creatures - a truly competitive congressional district. Republicans make up about 32 percent of the voters, Democrats 34 percent and independents 34 percent.

A mix of postwar suburbs and newer exurbs, the district is home to many retirees, health-care workers, professionals, and brewery, refinery and office workers.

In 2002, the district's voters sent Republican Bob Beauprez to Congress in the closest House contest in the country (a 121-vote margin out of 173,000 cast). In 2004, they reelected Beauprez but voted for John Kerry for president.

"It's like the judge was an evil genius," said Democratic candidate Ed Perlmutter, laughing as he examined a map of the district before going out to knock on voters' doors in Golden. "It's very middle American. It's suburban, it's right down the middle politically, it's right down the middle financially. It lends itself to very competitive races."

Perlmutter, a bankruptcy lawyer and former state senator, is running against Republican Rick O'Donnell, former state higher education chief. The incumbent, Beauprez, left the seat to run for governor.

"I'm not sure the district is an exact microcosm of the country, but it does reflect fairly well where a broad swath of the American public is right now," O'Donnell said, after talking with voters at a house party in Arvada. "Most Americans are pretty pragmatic."

A trend toward the middle has been growing in recent years, especially in the West and the upper Midwest. In states such as Wisconsin, Iowa, Minnesota, Montana, New Mexico, Arizona and Colorado, the politics of "red" Republicans and "blue" Democrats have given way to "purple" practicality.

"I think the pragmatism of candidates may be a step ahead of the party activists and some voters," said Lawrence Jacobs, chair for political studies at the University of Minnesota. "There is a cluster of voters, the ones you need to reach to win elections, that may be exercising a moderating force."

Jacobs cited Republican governors Tim Pawlenty of Minnesota, Mitch Daniels of Indiana, Owens of Colorado, and Democratic governor Jim Doyle of Wisconsin as moderate chief executives who have worked with the opposing party.

In the eight states of the mountain west, Montana, Wyoming, Arizona and New Mexico have replaced Republican governors with Democrats since 2002, and Colorado seems on the verge of joining them.

Despite the conventional shorthand of "red America" and "blue America," most Americans can't be characterized as either liberal or conservative, according to a survey released last month by the Pew Research Center. Pew concluded that "despite talk of 'culture wars' and the high visibility of activist groups on both sides of the cultural divide, there has been no polarization of the public into liberal and conservative camps."

Pew found Americans tended to be conservative on gay marriage, liberal on embryonic stem-cell research and a little of both on abortion. And, on abortion, the most divisive social issue, two-thirds of those polled said the nation needed to find a middle ground.

In Colorado's Seventh District, social issues such as abortion and gay marriage have generally been eclipsed by concerns about health insurance, immigration, education, and the war in Iraq.

O'Donnell, the Republican candidate, promised voters who gathered in the living room of an Arvada home that he would be a bipartisan representative. In a district where a third of voters are unaffiliated with either party, "voting against my party some actually helps me politically," O'Donnell said.

"The reason they're unaffiliated is that they think both parties stink," O'Donnell said. "They want to know that I'm not a party hack."

Perlmutter, though, has tried to capitalize on disenchantment with Bush by linking O'Donnell to the president. The war in Iraq and the president's unpopular veto of a stem-cell research bill are two favorite subjects, and Perlmutter also hammers O'Donnell for a 12-year-old proposal to abolish Social Security, which O'Donnell has since renounced.

"This year, there's a mood of 'we've had enough,' " Perlmutter said. "People are upset with Bush. They want a change of direction... . All of the voters appreciate the need for checks and balances, and we haven't had that for a while."

If there is a lesson for national politicians from Colorado's mood swing, Straayer says, it is this:

"Take a moderate, pragmatic line. Fix the problems with things like highways, schools and bread-and-butter issues. And get away from the moral stuff."