A powerful labor union sent a letter to Speaker Nancy Pelosi (D-Calif.) and Senate Minority Leader Charles Schumer (D-N.Y.) on Thursday announcing their opposition to bipartisan legislation to ban “surprise” medical bills patients can receive from hospitals and insurers when their services aren’t covered by insurance.

The International Union of Operating Engineers (IUOE), which represents more than 400,000 mechanics and machine operators nationwide, said it supports the effort to do away with surprise billing, but that the bipartisan legislation gaining traction in Congress would “impose devastating cuts to frontline medical providers and tilt the playing field in favor of insurers.”

“The proposals would give insurance companies outsized power to set artificially low reimbursement rates, reducing revenue that physicians and clinicians across the country depend on to keep the doors open,” union President James Callahan wrote to the Democratic leaders. “This policy would likely lead to physician shortages and even facility closures in many vulnerable rural and underserved communities, just as these critical providers are desperately needed.”

A bipartisan deal reached last year by the leaders of the House Energy and Commerce and the Senate Health, Education, Labor and Pension (HELP) committees would ban providers from sending the surprise bills, and would instead require insurers to foot the payments, with the costs benchmarked to the average price of the service.

A House committee aide disputed the union’s characterization, describing the bill as a middle ground between arbitration and price benchmarking. If the surprise bill is for more than $750, either party can bring it to arbitration, the aide said, but otherwise the provider will get paid the median in-network rate for that geographic region.

The aide noted that the legislation does not block states from having their own laws to address surprise billing claims.

Conservative groups have been raising alarms about the HELP legislation for months, saying it would implement “price controls” or “rate-setting” that gives the government too much domain over the private sector and would pave the way for a single-payer system.

Supporters of the legislation bristle at this allegation, arguing that the federal government will not be setting the price. The committee aide also said the legislation would not result in cuts to frontline medical providers, as Callahan claimed, but rather would level the playing field between insurers and patients.

Insurers support the legislation, which is backed by Energy and Commerce Committee Chairman Frank Pallone Jr. (D-N.J.), Rep. Greg Walden (Ore.), the top Republican on the committee, Senate HELP Committee Chairman Lamar Alexander (R-Tenn.) and Sen. Patty Murray (D-Wash.), the ranking member on the HELP panel.

The HELP bill is one of the few bipartisan pieces of legislation with a shot at getting to President Trump’s desk in an election year. Alexander and Walden are retiring at the end of the year, adding to the pressure they’re under to get the legislation passed.

Lawmakers tried to get the bill included in the 2019 end-of-year spending package, and then again into one of the coronavirus stimulus bills, without success.

The IUOE said it would support creating independent boards to mediate billing disputes, pointing to a model used in New York, that the union said has saved consumers $400 million dollars and reduced out-of-network billing by 34 percent since being implemented in 2015.

In the letter, the union bashed the insurers for supporting the HELP bill.

“This month, one major insurance company, United Health, beat its quarterly profit expectations as its stock price soared and its corporate executives continued to thrive,” Callahan wrote. “Meanwhile, hospitals and medical practices have been forced to cut hours, furlough healthcare workers, and, in some cases, close due to financial strain. There are better solutions to fix surprise billing that do not put the nation’s healthcare safety net or patients’ access to care at risk.”