JUNEAU (AP) — U.S. Sen. Mark Begich’s campaign is running radio ads taking aim at Charles Koch and David Koch, the billionaire backers of the conservative group Americans for Prosperity.

The group, in an ad that began running last month, said Begich is on record supporting a carbon tax and urging Senate Majority Leader Harry Reid to make it a priority. That claim stemmed from a 2010 letter Begich signed with 11 other senators.

The letter, which revolved around energy policy, included a line that read: “First, we believe the scale of this challenge dictates the need for a comprehensive solution that includes making polluters pay through a price on greenhouse gas emissions.” Americans for Prosperity, in announcing the ad buy, included a link to the letter with that sentence underlined.

Begich told reporters Monday that he does not support a carbon tax and did not agree with everything in the letter. He said his intent in signing was to make sure the Senate did not lose the discussion on climate change and a comprehensive energy plan.

Two fact-check organizations found the ad distorted or exaggerated Begich’s position.

Americans for Prosperity spokesman Levi Russell said Tuesday that the group is “very comfortable” with the contents of its ad and stands behind it.

Begich on Monday also took a swipe at the Koch brothers, which he said was an effort to provide “context” to who is behind the ad. Begich said they had no problem “firing” Alaskans and “leaving dirty water.” Flint Hills Resources, which is owned by Koch Industries Inc., recently announced plans to close the Flint Hills refinery at North Pole, citing the costs from the cleanup of the industrial chemical sulfolane as a major reason.

Begich, in his radio ad, refers to them as “billionaire outsiders” who turned their backs on Alaska.

Begich campaign spokesman Max Croes said the ad began running Tuesday and will run for three weeks.

Russell said Begich is attempting to spin away from the issue of the carbon tax to something unrelated.