PUEBLO, Colo.—In his six years in Congress, Democrat John Salazar has delivered on some key local priorities for his sprawling rural district.

Often working with Republicans, he blocked the Army from expanding a maneuver site across ranchland, pushed for a veteran's cemetery to be built, and secured federal funding for a water pipeline that had been stuck in the planning stage for 40 years. Even the chairman of the Colorado Republican Party said Mr. Salazar was well-liked and respected in the district.

Scott Tipton, the Republican candidate hoping to unseat Democratic Rep. John Salazar in Colorado's 3rd District, speaks at a rally in Pueblo. Chris Schneider for The Wall Street Journal

But in this autumn of discontent, where incumbents are taking heavy fire and Democrats are struggling to reconnect with an angry electorate, Mr. Salazar is fighting for his political life.

Mr. Salazar, a potato farmer by trade, is locked in a battle with Republican Scott Tipton, a state legislator who owns a pottery business. Four years ago, Mr. Salazar crushed Mr. Tipton, winning by 25 percentage points. The rematch, however, is a nail-biter.

A recent poll commissioned by the National Republican Congressional Committee—but also cited by Mr. Salazar's campaign—found the race tied at 45% each, prompting the Republican group to announce Wednesday that it was committing $700,000 to the race. An outside group that doesn't disclose its donors, Americans for Job Security, recently bought nearly $500,000 of ads attacking Mr. Salazar.

Republicans see the district, which stretches across Western Colorado, as fertile ground. Its voters backed the Republican presidential nominee in the last three contests and the Republican Party has gained more than 6,000 registered voters just since February, while Democrats have lost 700. Voter registration now stands at roughly 38% Republican, 33% Democrat and 29% unaffiliated.

Wearing blue jeans, a checked shirt and ostrich-skin cowboy boots, Mr. Tipton made a point of telling voters at a recent campaign stop here of his deep local roots. Then he pivoted to his campaign's central theme: tying Mr. Salazar to the unpopular Democratic leadership in Washington, D.C. Mr. Tipton repeatedly points out that Mr. Salazar has voted with House Speaker Nancy Pelosi about 97% of the time.

Mr. Salazar emphasizes that he broke with his party on several key issues, opposing the cap-and-trade energy bill and the bank bailout and calling for a year-long extension of the Bush tax cuts for all income groups. He has been endorsed by the National Rifle Association and rated a centrist by the National Journal.

"I've never voted with Nancy Pelosi," Mr. Salazar says, adding that "every vote I've taken" has been with and for his constituents.

John Salazar, left, beat Scott Tipton by 25 points in 2006. Associated Press

But the constituents who skipped a crucial high-school football game in Pueblo to talk with Mr. Tipton at a recent town-hall meeting weren't buying it. They were angry at what they see as runaway federal spending; at the health-care overhaul, which Mr. Salazar supported; and at a sagging economy.

"I can't vote for Mr. Salazar. He's spending too much money," said Yvonne Lopez, who described herself as a lifelong Democrat and retired union member. "I'm so sick of the way things are going right now," said Rick LaLonde, a case manager at a private prison.

Mr. Tipton echoed that anger. "The government has overtaxed, overreached, overregulated and we're paying the price," he said.

For his part, Mr. Salazar, an older brother of Interior Secretary Ken Salazar, has tried to paint Mr. Tipton as an extremist. He points out that earlier in the campaign, Mr. Tipton called for cutting government by 50%. Mr. Tipton now says he supports a 10% cut in all non-military discretionary spending. A spokesman called that "the first step" to fiscal responsibility but wouldn't say whether a 50% cut remains the goal."It's tough out there," Mr. Salazar said. "The way things are going, I could walk on water and be criticized for not swimming."

The two candidates have also clashed in increasingly nasty TV ads.

In one, Mr. Salazar calls his rival a "multi-millionaire banker" and implies he profited from the bank bailout. Mr. Tipton did sit on the board of a financial institution several years ago—as did Mr. Salazar—but didn't have direct involvement with the bailout. As for the label "multi-millionaire," Mr. Tipton acknowledges his small business has been successful. "We've done OK," he says. "I hope that's not a sin in America."

Mr. Salazar, meanwhile, has come under fire in $490,000 worth of ads purchased by an outside group, Americans for Job Security, which does not disclose its donors. The spot depicts a burglar pilfering a home, then likens Mr. Salazar and Ms. Pelosi to thieves.

Write to Stephanie Simon at stephanie.simon@wsj.com