Congress is getting close to advancing one of the few significant bipartisan reforms to the health care system still on the docket: legislation to curb the practice of “surprise medical billing.”

Naturally, that progress has sparked a last-ditch, dark money blitz bent on sinking these relatively modest bills, which aim to make it harder for unsuspecting patients to get hit with exorbitant bills if they see the wrong doctor in an emergency situation.

A group called “Doctor Patient Unity,” formed in June, has bankrolled a sweeping campaign of radio and television ads to pressure senators up for re-election in 2020 to oppose proposals to reform surprise medical billing.

Now, the dark money group is going after key lawmakers with direct appeals to their constituents. A mailer paid for by Doctor Patient Unity, obtained by The Daily Beast, urges Rep. Tim Walberg’s (R-MI) constituents to call his office and tell him to “say no to rate-setting” and “to put patients first.”

Walberg, a member of the House Energy and Commerce Committee that approved the so-called “No Surprises” reform bill in July, has spoken out against surprise medical billing in the past. The mailer claims that the congressman “can stop big government and insurance companies from controlling your health care” and argues that billing reforms are “the first step towards Socialists’ Medicare-for-All dream”—a possibly resonant line of attack in the congressman’s traditionally Republican district.

Doctor Patient Unity has already dropped millions of dollars on television and social media ads, according to documents on file with Facebook and the Federal Communications Commission (FCC). But the emergence of a direct-mail campaign—traditionally among the most expensive ways to reach voters—represents a major escalation in the efforts of groups fighting to maintain the status quo on medical billing. And that’s ringing alarm bells on Capitol Hill among those who see the reform push as perhaps their best chance at meaningful action to improve the health care system until the next congressional session begins in 2021. Congressional aides told The Daily Beast that lawmakers from both parties with jurisdiction over health care legislation have been targeted with similar mailers.

"They’re throwing the full kitchen sink at our efforts here and seeing what will stick," said a congressional aide, who requested anonymity to discuss the matter candidly. “It’s unclear whether or not members are seeing a huge influx of constituent calls, but it’s obvious what they’re trying to do here, which is to back members off this proposal.”

Recent scrutiny of hospital billing practices from advocates and the press has made surprise billing a major area of political concern. In May, President Trump called on Republicans and Democrats to address the issue, saying “we’re determined” to end the practice.

Victims of surprise billing—estimated to be one out of every six Americans—have received six-figure bills after being treated for heart attacks, five-figure bills for routine post-surgery tests, and smaller but still considerable invoices for other kinds of emergency treatments. These charges are frequently incurred when someone goes to a hospital that’s in their insurance network but is treated by a doctor who is not—often a specialist like an anesthesiologist.

Lawmakers in the House and Senate are coalescing around proposals that would set a cap for those out-of-network charges at the rates charged by insurance companies in that area. Hospitals and some physician groups are alarmed that such a change could put themselves on the hook to lose more money in providing treatment.

Those hospitals and doctors have led the charge in publicly opposing the changes Congress is mulling. But as for who exactly is pouring money into the mailers and TV ads, Doctor Patient Unity appears to have taken pains to conceal the identities of the individuals or organizations running and financing the effort. Public records provide some clues, however. Incorporation documents on file in Virginia list an address in the town of Warrenton that is shared by the prominent Republican law firm Holtzman Vogel Josefiak Torchinsky.

The group’s treasurer, according to filings with the FCC, is a woman named Janna Rutland. She serves as treasurer for a handful of other political and policy organizations, and appears to be an employee of the GOP consulting firm Crosby Ottenhoff, which counts a number of high-profile Republican candidates and party organs among its clients.

While Doctor Patient Unity and its funders are waging an all-out war to move lawmakers and public opinion—if not just muddy the waters on the issue—the targets of its campaign don’t seem to be spooked.

A spokesman for Walberg, one of the lawmakers hit with a mailer, said the Republican is committed to working on bipartisan solutions to surprise medical billing “regardless,” he said, “of what any dark money group may claim.”