(CNN) - Sen. Mark Begich is taking aim at the Koch brothers.
The first-term Alaska Democrat, who faces a challenging re-election this November, has been targeted in two TV commercials put up by Americans for Prosperity, a conservative group backed by the deep pockets of the billionaire industrialist brothers, David and Charles Koch.
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Now Begich is pushing back, slamming those ads in a new commercial, the first TV spot of his re-election campaign.
"First it was a D.C. actress pretending to be an Alaskan. Now ads attacking Mark Begich on a carbon tax have been called false and not true. Who's behind the attacks," says the narrator in the ad.
The commercial goes on to showcase Alaskan voters who are critical of the Kochs, and attacking them for what they say is the closing of an oil refinery owned by Koch Industries.
"They come in to our town, buy our refinery," says one man.
Another man says the company is "just running it into the ground," with another saying it's "leaving it a mess."
The spot concludes with another man saying "I don't go down and tell them what to do; I don't expect them to come to Alaska and tell us what to do."
A Democratic source with knowledge of the Begich ad buy confirms to CNN that the commercial, first reported by Politico, is running starting Monday, in the Anchorage and Fairbanks markets.
A spokesman for Americans for Prosperity pushed back against Begich.
"Senator Begich has yet to give a coherent answer on why he pushed for a price on greenhouse gas emissions, or voted twice to pave the way for a carbon tax. With Alaskans paying some of the highest energy prices in the country, it would be nice to see Begich be honest and stop trying to distract from his record," AFP Communications Director Levi Russell told CNN.
AFP has spent around $850,000 to run their two TV commercials in Alaska. The group has been the biggest spender to date this election cycle, shelling out nearly $30 million since October to run ads, most of them focused on health care, that attack Democrats and praise Republicans.
Democrats hold a 55-45 majority in the Senate (53 Democrats and two independents who caucus with the party), but are defending 21 of the 36 seats up in November, with half of those Democratic-held seats in red or purple states, like Alaska.
Senate Majority Leader Harry Reid has repeatedly attacked the Koch brothers as being "un-American" and having "no conscience," and he's argued that the "vast majority" of the AFP ads are untrue.
AFP President Tim Phillips said recently that Reid is using scare tactics against detractors.
"For a politician to try and silence and intimidate an American citizen solely because they don't like the facts that are coming out and the story they are telling says more about the politician than it says about Americans for Prosperity," Phillips told CNN's Leigh Ann Caldwell in a sit-down interview near his office in Arlington, Virginia.