Sen. Mark Udall on Monday apologized for saying that two reporters beheaded by militants in the Middle East would agree that the United States should not "be impulsive" in responding to their deaths.

Udall, a Democrat, made the comments in an untelevised debate with challenger Rep. Cory Gardner on Saturday night in the western Colorado town of Grand Junction. Udall has been criticized by Republicans for saying the Islamic State of Iraq and the Levant, which currently controls much of Iraq's heartland, is not an imminent threat to the United States. At the debate, he argued that the two American journalists whose beheadings it filmed and released would agree with him.

"Steve Sotloff and James Foley would tell us, don't be impulsive," Udall said. "Horrible and barbarous as those executions were, don't be impulsive, come up with a plan to knock ISIL back."

Video of the exchange first surfaced Monday on the conservative web site National Review Online.

Gardner, a Republican, did not respond to the statement at the debate. But on Monday he slammed the incumbent. "It's outrageous that Senator Udall would put words into the mouths of dead Americans," Gardner said in a statement.

Udall apologized for the remark less than an hour later. "I should not have invoked the names of James Foley and Steven Sotloff," he said in a statement. "It was inappropriate and I sincerely apologize."

Colorado's senate race is one of the most competitive in the country. Recent polls have shown Gardner and Udall neck-and-neck. Republicans need to net six senate seats in November to win control of the chamber.