The group quickly grew to more than 1,800 industry members. It lobbied statehouses and regulators and went public with some of the sport’s dirtiest laundry to try to shame horsemen, veterinarians, politicians and regulators into treating thoroughbreds like athletes rather than commodities filled with a smorgasbord of anti-inflammatories and painkillers.

The group, along with the Jockey Club, helped shape a federal bill introduced in March with bipartisan support to create a uniform national standard for drug testing and medication rules in racehorses that would be overseen by the United States Anti-Doping Agency. The bill is backed by owners and breeders’ associations, animal welfare groups and racetracks, including the Stronach Group, which owns Santa Anita Park, and the New York Racing Association, the hosts of the Preakness Stakes and the Belmont Stakes.

Notably, Churchill Downs Inc., the host of America’s most famous race, and Senator Mitch McConnell, Republican of Kentucky and the Senate majority leader, who counts the company among his top patrons, do not support the bill.

“He politely and candidly told us that he appreciated what we were trying to do, but until Churchill got involved he couldn’t push the bill,” said Hancock, who said he met with the senator in 2016. McConnell, through a spokeswoman, declined to comment.

In a statement, Bill Carstanjen, chief executive of Churchill Downs, said the company had reviewed the bill and requested changes that were not addressed. “We don’t believe a federal bill is practical, reasonable, or imminent,” he said.

In a memo obtained by The New York Times that lays out Churchill Downs’s opposition to the bill, the company cites costs and the fact that trainers and veterinarian groups are against the measure. It also asks whether having the support of the Humane Society of the United States is “letting the fox in the henhouse.”

The company also points to the antidoping agency’s role in bringing down the cyclist Lance Armstrong, suggesting Usada might find much wrong and could not be trusted to keep information out of the public eye.