Coal miners in southeastern Ohio are blasting an ad from President Obama that claims they were forced to “be props in (Mitt) Romney’s commercial.”

Romney appeared at an Aug. 15 event in Belmont County surrounded by 500 coal miners, and claimed that unnecessary regulation by the Obama administration’s Environmental Protection Agency amounts to a “war on coal,” and has helped put thousands of miners’ jobs in jeopardy. The Obama campaign countered with an ad charging the miners were forced to attend the Romney rally, and that they lost a day’s pay.

That ad followed a mining company official’s acknowledgement that workers were told attendance was “mandatory” — though he claimed they weren’t really “forced” to attend.

A group of 500 miners has now penned a letter and released a video insisting that no one was forced to attend the rally and demanding that the Obama campaign disavow the ad. The workers at Century mine, in Beallsville, Ohio, which is operated by Murray Energy subsidiary American Energy Century, said they “voluntarily and enthusiastically” attended the event.