With Primary Day being held Tuesday, PolitickerCO.com takes a look at the four competitive congressional primary races in Colorado.

1ST CONGRESSIONAL DISTRICT

Incumbent: Diana DeGette (D-Denver)

Candidates:

• Charles Crain: the 2000 Republican candidate from Denver
• George Lilly: the 2004 American Constitution Party candidate from Denver

Overview: It's the most insignificant competitive congressional primary in the state this year, as neither candidate is strong enough to beat DeGette in November. Lilly at least has an organization (and, unlike Crain, a website and enough money to require Federal Election Commission report filings), and he beat Crain with about 61 percent of the vote at the 1st District GOP assembly. But he's has alienated many mainstream Republicans by speaking out against the GOP "establishment" and vocally supporting Ron Paul. Crain is running to help Bob Schaffer defeat Mark Udall in the U.S. Senate race -- Crain particularly dislikes Udall's record on energy. But Crain, whose registered address is at a Denver homeless shelter, has little name recognition and even less money.

What to look for: A Lilly win and low voter interest.

2ND CONGRESSIONAL DISTRICT (Democrats)

OPEN SEAT: U.S. Rep. Mark Udall (D-Eldorado Springs) is running for U.S. Senate

Candidates:

• Joan Fitz-Gerald: Former state Senate president from Coal Creek Canyon
• Jared Polis: Entrepreneur and philanthropist from Boulder
• Will Shafroth: former environmental group executive director from Boulder

Overview: With about $10 million spent by the three candidates, it's the most expensive primary in Colorado history. Fitz-Gerald, the more mainstream candidate, should do well in the sparsely populated western mountain counties, having represented them in the state Senate. Polis' campaign was slow out of the gate, but after a staff reshuffle it has returned strong, fueled by massive ad buys. Shafroth has a strong base of environmental voters to rely on.

What to look for: Turnout. The more ballots cast, the better Polis will likely fare. It's worth looking at how Adams County, home to a third of the district's population, will vote.

5TH CONGRESSIONAL DISTIRCT (Republicans)

Incumbent: U.S. Rep. Doug Lamborn (R-Colorado Springs)

Other Candidates:

• Jeff Crank: Businessman and former Congressional staffer from Colorado Springs
• Bentley Rayburn: Retired Air Force major general from Colorado Springs

Overview: In this 2006 rematch, Lamborn has mostly been content to let Crank and Rayburn duke it out among themselves about which of them should drop out of the race. Crank has been hitting Lamborn hard in ads and has an excellent grassroots team. Lamborn has solid support among social conservatives (absolutely vital for any 5th Congressional District candidate to have), but his lackluster fundraising numbers this election cycle have made him look vulnerable. Rayburn needs a good turnout from military veterans and unaffiliated voters to hang in there.

What to look for: Absentee ballots. In 2006, Lamborn prevailed on the strength of mail-in ballots -- especially during the first couple weeks mail-in ballots were available. This year, Republican mail-in ballot requests have doubled, though the return rate has dropped significantly.

6TH CONGRESSIONAL DISTRICT (Republicans)

OPEN SEAT: U.S. Rep. Tom Tancredo (R) is not running for re-election

Candidates:

• Wil Armstrong: Businessman from Greenwood Village
• Mike Coffman: Colorado Secretary of State from Aurora
• Ted Harvey: State Senator from Highlands Ranch
• Steve Ward: State Senator from Littleton

Overview: The race appears to be between Armstrong and Coffman, and observers say Coffman has the edge. Armstrong, the son of former U.S. Sen. Bill Armstrong, has been endorsed by the Who's Who of Colorado Republicans. Coffman has a good district-wide organization and, as Secretary of State, has significant name-recognition. Harvey can rely on support from Douglas County and the most conservative (and most reliable) GOP voters. Ward's base of support is largely limited to Arapahoe County.

What to look for: Whether Armstrong can get late-breaking support. Coffman won the early edge in the absentee ballot race. The Armstrong campaign cited a poll in late July showed him pulling him up to Coffman, but can Armstrong secure enough votes on Tuesday and from last-minute absentee voters to overcome Coffman's early advantage? Turnout will be crucial to Armstrong's chances, especially since he sinks or swims on the undecided vote.

ALSO on PolitickerCO.com:

• 2008 primary preview: State House races
• 2008 primary preview: State Senate races