The U.S. Chamber of Commerce, a pro-business group, is coming to the aid of Senate Minority Leader Mitch McConnell with an ad released Monday that calls the embattled Republican a protector of the coal industry and "a fighter who never lets Kentucky down."

"As the EPA is trying to kill Kentucky's coal industry, Mitch McConnell is fighting back, fighting hard," a narrator says in the ad, referring to new Obama administration emissions standards that are hitting existing coal plants hard.

Regulation of the coal industry, among Kentucky's largest economic drivers, promises to be a prominent issue in McConnell's challenging re-election bid next year against Democratic Secretary of State Alison Lundergan Grimes, who has been portraying herself as friendlier to coal than McConnell.

The Chamber of Commerce's new ad also signals the latest attempt by pro-business Republicans to counter the influence of conservative groups like the Senate Conservatives Fund, which is bankrolling a Republican to challenge McConnell in a primary. McConnell has publicly taken the SCF to task, arguing the group is undercutting viable Republican Senate candidates while promoting ideological purity for its own profit. Meanwhile, the SCF has said it is not concerned with winning a Republican majority in the Senate, but with more conservative candidates winning seats.

The chamber has not shied away from engaging in Republican primary contests in the past, but it has vowed to do so with more gusto during the 2014 midterm elections, in which conservative groups were already threatening to run conservative challengers against long-time incumbent Republicans.

The chamber saw early success in a special election for a House seat in Alabama, where it recently spent money to help lead the more moderate Republican to victory in the primary race for a safely Republican seat.