After a year of backroom, closed-door, GOP-only meetings on health care, and a bitter, partisan floor fight over repealing the Affordable Care Act that eventually collapsed, senators from both parties came together to hold nearly half-a-dozen public hearings and hammer out a bill to stabilize Obamacare’s vulnerable insurance exchanges by the end of September.

But beneath the bipartisan bonhomie, there is trouble in paradise.

In exchange for funding Obamacare’s subsidies to insurers that cover care of low-income people with severe health needs, Republicans are demanding that some of the ACA’s protections and mandates be waived—and have suggested rolling back the requirement that every insurance plan cover essential health benefits like maternity care and mental health treatment.

This week, Sen. Lamar Alexander (R-TN), the chair of the committee crafting the stabilization bill, mocked the concept of forcing all insurers to offer comprehensive health care plans.

ACA waiver offers states as many options on benefit design as a menu with only 1 item or a Dr. Seuss book titled “Oh, The Place You’ll Go” — Sen. Lamar Alexander (@SenAlexander) September 12, 2017

Senate Democrats say they’re open to some increased “flexibility” but worry that allowing too many regulatory rollbacks will lead to more expensive and worse quality health coverage for millions of people. Though many proposals have been tossed onto the table over the past few weeks, the key battle is currently over loosening the rules around Obamacare’s “state innovation” waivers—which states obtain from the federal government to test different health care systems. Some states, including Alaska, have used these waivers to set up reinsurance programs, which have brought down the number of uninsured residents and lowered costs. But other states are seeking waivers for plans that health care experts say would create “barriers to enrollment.”

“Flexibility needs to increase health care for people, not decrease it. If it’s flexibility to take health care away, that’s not something I would support,” Sen. Debbie Stabenow (D-MI) told TPM. “I believe that health care is a basic human right and we’re going to make sure that everyone has health care.”

Asked if she believes her Republican colleagues’ proposals would decrease coverage, Stabenow responded: “It’s just not clear at this point.”

During simultaneous Senate hearings on health care Tuesday morning, Republican senators and their conservative guest speakers proposed scrapping or loosening a host of Obamacare’s core provisions—from age ratings that limit how much insurance companies can charge older patients, to the individual mandate, to the requirement that all plans cover essential health benefits.

Without a concession or a win they can point to, GOP aides and lawmakers have said, it would be hard to get their caucus on board with funding the cost-sharing reduction subsidies or taking other steps to prop up Obamacare’s marketplaces.

“To get a Republican president and a Republican House and a Republican Senate just to vote for more money won’t happen in the next two or three weeks unless there’s some restructuring,” Alexander said.

But Democrats on those committees remain staunchly opposed to the suggested changes, and said the “guardrails” built into the ACA must be preserved at all costs. The two sides are currently at an impasse.

“Negotiations are still moving, but it sounds like Senator Alexander is having a bit of trouble with the extreme wings of his caucus,” a Democratic staffer told TPM. “We’re continuing to work with him, but we’ve been clear that anything that the Republicans want to do that would allow insurance companies to provide less or worse coverage—that’s not something we’d be willing to do.”

The staffer confirmed that Republicans are pushing for the weakening of Obamacare’s essential health benefits rule, particularly the maternity care mandate.

“That’s why it’s so important that [Sens.] Collins and Murkowksi are on the HELP committee,” the staffer noted. “They’re committed to fighting for women’s health, so it’ll be hard to get the two of them on board for anything that cuts or guts it.”

Testifying before the Senate on Tuesday, former Health and Human Services senior counselor Aviva Aron-Dine warned lawmakers against against attempting to revert to a system where patients only pay for the exact health care services they personally require.

“A la carte health insurance doesn’t work,” she said. “If you say that plans don’t have to cover mental health, that’s not actually a choice for people about whether to buy it, because in practice, only the people who need those services will buy it, and then the cost becomes unaffordable and it’s no longer insurance. There is a fundamental need for pooling.”

The health care negotiations—which are occurring during an insane crunch time for Congress—have an additional stress factor: President Donald Trump’s repeated threat to cut off the CSR payments to insurers.

Though Trump committed to the August installment of the subsidies, he has not yet confirmed whether he will come through in September. Cutting off the payments, the non-partisan Congressional Budget Office reported, would raise insurance premiums by 25 percent and would lead to millions of people having no insurer at all. Senators on both sides of the aisle have pushed for Congress to appropriate these payments for at least another year to ensure more stability and take away Trump’s power to blow up the individual market. But even if Congress meets the ambitious timeline of passing a bill by the end of the month, the earliest they could appropriate the payments would be October, giving Trump one last chance to wreak havoc.

Several lawmakers also expressed fears that the bipartisan negotiations around Obamacare stabilization could be stymied by a last-ditch effort by Senate Republicans to attempt to repeal much of Obamacare before the clock runs out on their ability to pass it with only 50 votes.

“Republicans and Democrats are finally working together here, and it is refreshing, so it would be deeply disappointing if another partisan debate about Trumpcare erupted and derailed our efforts here,” Sen. Patty Murray (D-WA) complained Tuesday. “I hope those senators will join our conversations instead of doubling down on harmful repeal efforts that people across the country have rejected.”