When WME/IMG bought the Ultimate Fighting Championship for $4 billion, it signaled that it sees tremendous upside for the global expansion of the sport. But the agency is also taking on risks, as some fighters are waging a struggle for greater power in the sport, which could pose a threat to the UFC’s business model.

One of the key figures in the fight is Markwayne Mullin, a former MMA fighter who is now a Republican congressman from Oklahoma. Mullin has proposed legislation that would force the UFC to share financial information with fighters and create an independent ranking system, both steps that would give greater leverage to fighters. The UFC has been adamantly opposed to the bill, called the Ali Expansion Act.

“Maybe this new group will see the importance of the athlete to make sure the sport is sustainable,” Mullin told Variety on Tuesday. “The UFC has treated fighters like they were a commodity.”

Mullin used to fight both in the cage and in the Octagon. He says he never made very much money, and after paying for his own surgeries he probably didn’t break even.

The UFC has grown in leaps and bounds since then. Once considered a brutal fringe sport, it has become mainstream and tremendously profitable, and is poised to expand in Europe and Asia. But many fighters allege that success has come at the expense of fighters. For years, they have complained about long-term contracts, a rankings system controlled by the UFC and low pay among the lower ranks of performers.

“The minimum wage in the NBA is $500,000,” said Nate Quarry, a former UFC fighter who is now a plaintiff in a class-action suit against the promotion. “The athletes in the UFC have no minimum wage. They have no rights to negotiate.”

Some have banded together to form the Mixed Martial Arts Fighters Association, which some see as a precursor to a union. The class-action lawsuit accuses the UFC of being an illegal monopoly and seeks to open up contracts to allow for greater competition. And then there’s Mullin’s bill, which would extend regulations that apply to boxing as well as MMA, creating an independent ranking system and giving more leverage to the fighters.

None of these issues was seen as serious enough to prevent WME/IMG from paying a record-setting price for the UFC. But as the sport reaches “the next level,” as UFC president Dana White put it when announcing the sale, some say it will have to take a fresh look at labor relations.

Erik Magraken, an attorney who blogs at CombatSportsLaw.com, says the sale may finally give the fighters’ association some teeth.

“If I’m a fighter and I’m reading the headlines of the $4 billion valuation, my next call would be to the fighters’ association to get on board,” he said.

If he were conducting due diligence for the buyers, he said he would be most worried about “fighters demanding an objective revenue split consistent with other major sports.”

WME/IMG declined to comment for this story. The sale will not be final for several more weeks. White will stay on as president, so any changes may be very slow in coming.

The UFC has been adamantly opposed to Mullin’s bill, which does not seem to pose a risk of becoming law anytime soon. But Mullin, the only member of Congress who is a former MMA fighter, argues that giving more power to fighters is in the sport’s best long-term interests. With more ability to negotiate, fighters would have longer careers and would attract larger fanbases, he says.

“The only way the sport is sustainable is the fighters being treated on an even playing field,” Mullin says. The bill, he said, “is great for UFC. It doesn’t spit up athletes and throw them out. That’s what happens right now. Athletes are here today and gone tomorrow… This is about sustaining the sport for the promoter and the athlete.”