Gardner said in a statement that serving in the Senate is a "tremendous honor."
Colorado Republican Cory Gardner is seeking to oust Democrat Mark Udall in one of a handful of key U.S. Senate races.
A super PAC operated by the Ricketts family is the latest outside group duking it out on TV in one of the hottest U.S. Senate races in the country.
See where U.S. Senate candidates Democrat Mark Udall, Libertarian Gaylon Kent, and Republican Cory Gardner stand on a range of issues.
Colorado's Senate candidates struggled to pin the other down on energy policy and reproductive rights Wednesday in their fifth and final debate.
Sen. Mark Udall is enmeshed in a tight race against Rep. Cory Gardner, and polls have shown the contest deadlocked for months.
Control of the Senate could hinge on the outcome of the race, which has seen six months of bruising attack ads and remains deadlocked in public polls.
The debate was the second of three scheduled for this week, making it an important stretch in a pivotal campaign.
The nonprofit group founded by Karl Rove is spending $9.7 million in an effort to defeat incumbent Democratic Sen. Mark Udall.