A conservative watchdog group asked the Office of Congressional Ethics on Thursday to review whether 14 Democratic lawmakers and one Republican inappropriately used their office to try to coerce a Las Vegas casino mogul into allowing his employees to unionize.

In May, Wisconsin Rep. Mark Pocan and 13 other Democrats wrote a letter to Frank Fertitta III, the CEO of the Las Vegas-based casino management company Red Rock Resorts Inc., urging him to “respect the rights of employees” at the company “to form a union and collectively bargain.”

Republican Rep. Brian Fitzpatrick of Pennsylvania sent Fertitta a similar letter the following day.

Both letters mentioned a pending piece of legislation — co-sponsored by most of the signatories — that would tweak a part of the 2017 GOP tax code overhaul that inadvertently more than doubled the depreciation period for renovation and remodeling projects.

Red Rock had written to Congress in December asking it to correct the “drafting error” on the renovation depreciation schedule in the 2017 tax overhaul. In the letters, the lawmakers say the bill would impact tax implications for Red Rock’s then-recently completed $690 million renovation project on the Palms Casino Hotel in Las Vegas.