A new radio ad from a conservative group says Democrats are "race-hustling" in the North Carolina Senate matchup, an accusation that could roil one of the year's most competitive contests.

The controversy started with a radio ad from the Senate Majority PAC, a super PAC allied with Democratic Senate Leader Harry Reid. That ad, aimed at black voters, says GOP candidate Thom Tillis opposes equal-pay-for-equal-work legislation and raising the minimum wage, and that he "made it harder for communities of color to vote."

It then invokes the name of Trayvon Martin, the Florida teenager killed earlier this year in a case that sparked a national discussion around stand-your-ground laws.

"Tillis even led the effort to pass the type of stand-your-ground laws that caused the shooting death of Trayvon Martin," a narrator says in the Democratic ad. The minute-long spot then praises Hagan "because she believes in standing up for all North Carolinians."

While Senate Majority PAC did not announce its radio ad in the North Carolina race, conservative media caught it.

And in response, American Commitment, one of the lesser-known organizations linked to the Koch Brothers, replays a clip of the Democratic spot while its own narrator bashes the content. "Have you heard this race-hustling Kay Hagan ad paid for by Harry Reid's Super PAC?" a narrator says. "Probably not. Because they're not running it on this station."

The narrator then pauses for the part that mentions Martin before starting up again. "Desperate to hold onto power, Obama, Reid, and Hagan are shamelessly race-baiting. And it's only going to get worse in the final weekend."

The American Commitment ad praises Tillis's "bold" leadership as the speaker of the state House before asking voters to not let "the race-hustlers win. Vote Thom Tillis on Tuesday."

The group is spending $30,000 to run the ad on Tuesday and Wednesday—a small buy in a race that has seen more than $100 million in spending. But American Commitment spokesman Stephen Manfredi said the group could increase its buy in the coming days

American Commitment is something akin to the Koch networks' pit bull, far less well-funded than counterparts like American for Prosperity but willing to engage fiercely with ideological foes. It's made a mission in recent months of personally targeting Democratic mega-donor Tom Steyer, a wealthy environmentalist who has funded a series of TV ads in battleground Senate races.

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