The congressional candidates' debate was supposed to be about foreign policy and the economy, but Democrat John Salazar started off by offering a farming tip.

"Being a potato farmer, I've learned that if you want to increase your harvest, you have to rotate your crops from time to time," the plain-spoken Salazar said. "Well, now it's time for Colorado to rotate its congressional crop, so we can get more of a harvest out of our representatives."

For most of the United States, there is almost no rotation of the political crop anymore in elections for the U.S. House. Politicians have created such extensive incumbent-protection mechanisms that more than 95 percent of the lawmakers who run for reelection routinely win. And most districts have been designed to be one-party monopolies, so there is rarely a party shift even when a seat opens.

But Colorado this year seems to be in crop-rotation mode. In two of the state's seven districts, Democrats have a chance to pick up seats from the GOP, which dominates the House delegation 5 to 2 going into the Nov. 2 election.

One of the tossup races here is no surprise. The 7th Congressional District, a suburban horseshoe curving around Denver, was drawn by a state judge specifically to produce competitive elections. With one-third of the voters Republican, one-third Democrat and one-third unaffiliated, the 7th had the closest congressional contest in the country in 2002.

Republican Bob Beauprez won the new seat by 121 votes out of 164,000 cast that year. This fall, he's in another tight race. A well-known challenger, Jefferson County District Attorney David J. Thomas, is benefiting from a strong Democratic voter-registration effort.

But the Democrats' best shot at taking back a congressional seat appears to be in the huge 3rd District, a sparsely populated swath of mountain and valley (the district is larger than the state of Virginia) that takes in the city of Pueblo and most of the western half of Colorado.

With popular Rep. Scott McInnis (R) retiring to enter the private sector, the Democrats nominated Salazar, a member of a well-known local Hispanic family. The candidate's brother, Ken, is Colorado's attorney general and the Democratic nominee for the U.S. Senate this year. The brothers appear to be helping each other's campaigns. "We just say, vote for the Salazar nearest you," the Senate Salazar laughs.

Third District Republicans had a tougher time choosing their candidate. Former state environmental director Greg Walcher won the nomination after a recount in a bitterly contested five-candidate primary.

Walcher, smooth and articulate on the stump, positioned himself to the right of the field in the primary, running against "activist judges" and "liberal special interests." He repeats those themes in the general-election campaign, with particular emphasis on his support for a constitutional amendment banning same-sex marriage. But it is not clear that is a winning position here. In a region that generally prefers to limit governmental control over personal decisions, many of the district's most prominent Republicans, including McInnis and U.S. Sen. Ben Nighthorse Campbell (R), have come out against the amendment.

Salazar's strongest issue is water, a precious commodity in a farming district that gets less than 12 inches of rain per year.

The potato farmer won statewide prominence in 2002 when he campaigned hard against a referendum to finance new water projects. Salazar argued that the vote was really a gambit to allow thirsty Denver suburbs to suck water out of the western half of the state.

But Walcher, following the lead of his boss, Gov. Bill Owens (R), supported the water project. The referendum lost. Today, Salazar never misses a chance to remind voters that his opponent "stood on the side of the urban interests while I was fighting for the rural areas here in the 3rd District."

The argument seems to be working. Recent polls have shown Salazar with a double-digit lead as the campaign entered its final month.

In Colorado's new 7th District, created after the 2000 census, Beauprez has taken full advantage of the built-in benefits of incumbency, blanketing the district with mailings and bringing in prominent figures to raise money and support.

But Thomas, the Democratic challenger, has run on basic economic issues, charging that Beauprez has sided with industrial interests that give him contributions. "We need jobs in this district," Thomas says in a television ad, "not tax breaks for companies that shift our jobs overseas."

Beauprez has run a largely negative campaign, with ads blasting Thomas's performance as district attorney. A particular problem for the Democrat is the charge that he may have been involved in a coverup by law enforcement officials after the Columbine High School massacre in 1999. Thomas denies any wrongdoing, but Columbine remains a potent topic.

"You'd tend to think Beauprez has an advantage because of incumbency," Denver pollster Floyd Ciruli noted. "And Columbine could be a problem for Thomas. But the registration figures this fall show several thousand new Democratic voters. That's substantial in a district as close as the 7th."

To help Beauprez hold the seat, the White House and GOP leaders in the Colorado legislature pushed a "re-redistricting" plan last year to redesign the district and make it safely Republican. That effort was blocked by the state Supreme Court; the Colorado 7th remains one of the nation's most competitive congressional districts.

In the 4th Congressional District, a farming area in the state's northeast corner, first-term Rep. Marilyn Musgrave (R) has proven a potent fundraiser among conservative groups because she is the House sponsor of the proposed amendment banning gay marriage.

The Democrats recruited a popular former state senator, Stan Matsunaka, to run against her, promising that he would get big contributions from liberals eager to defeat Musgrave. But polls suggest Musgrave will easily withstand the challenge in the strongly Republican district.

Republican Greg Walcher, left, and Democrat John Salazar debate in the 3rd Congressional District race.