The National Rifle Association said Wednesday the group is unequivocally opposed to the newly-struck compromise plan to expand background checks — and threatened that it may seek to penalize lawmakers who vote for what it deems “anti-gun” measures by giving them poor grades in their rating system.

The warning to members of Congress came just hours after a compromise on expanding background checks for gun purchasers was announced, a deal that the NRA itself participated closely in.

"Expanding background checks, at gun shows or elsewhere, will not reduce violent crime or keep our kids safe in their schools," top NRA lobbyist Chris Cox wrote in a letter sent to senators Wednesday night. "Given the importance of these issues, votes on all anti-gun amendments or proposals will be considered in NRA's future candidate evaluations."

The NRA rates lawmakers based on how they vote on the group's priorities. The letter grades are highly influential and carry particular weight in rural states with a strong gun culture.

Cox was a ubiquitous presence during negotiations between Sen. Joe Manchin, D-W. Va., and Pat Toomey, R-Pa., who announced the deal Wednesday morning to expand background checks to sales at gun shows and over the Internet.

Manchin has been careful to court the group's support; both he and Toomey both have "A" ratings from the NRA.

But despite their involvement, the NRA ultimately decided to come out against it.

Earlier Wednesday, a Senate Democratic aide had said that Democratic leaders were operating under the impression that the NRA will not throw its full weight behind opposing the background check bill, something which would have relieved pressure on moderate Democrats and Republicans to vote for the legislation.

Now, they have less cover.

Separately Wednesday, Sens. Patrick Leahy and Susan Collins announced they'd reached an agreement with the NRA on gun trafficking language that will be included in the overall gun bill. Cox did not mention the gun trafficking measure in his letter.

NBC News’ Luke Russert contributed to this report.