Republican incumbent: Wayne Allard (not seeking reelection)
Republican candidate: Bob Schaffer
Democratic candidate: Mark Udall
Schaffer, 46, was a congressman from 1997 to 2003. He served in the state Senate from 1987 to 1996.
Udall, 58, has represented Colorado in the U.S. House since 1999, after one term in the state House of Representatives. I gave more details about the Udall dynasty in this post about his cousin Tom, a Senate candidate in New Mexico.
Polls cited by Real Clear Politics show a single-digit lead for Udall.
This seat has been held by Republicans for 30 years. However, the other Colorado seat has had more of a mixed recent history. Ben Nighthorse Campbell was elected as a Democrat in 1992, but switched parties in 1995. The current junior senator is Democrat Ken Salazar, who was elected in 2004.
There has been a Democratic trend in Colorado this decade, and much attention will be paid to whether Udall can maintain his slim lead, and gain another victory for his party there.