But Republicans are learning the limits of the fast-track rules they are using. The Senate is considering the repeal bill under special procedures that preclude a Democratic filibuster, but the procedures also limit what can be included in the bill.

“The function of reconciliation is to adjust federal spending and revenue, not to enact major changes in social policy,” said Senator Bernie Sanders, independent of Vermont. “The parliamentarian’s latest decision reveals once again that Republicans have abused the reconciliation process in an attempt to radically change one-sixth of the American economy by repealing the Affordable Care Act.”

The Senate bill would give states sweeping new authority to opt out of federal insurance standards established by the Affordable Care Act. It builds on a section of the law that allows states to obtain waivers for innovative health programs. But it would relax many of the requirements for such waivers that Democrats put into the law, signed by President Barack Obama in 2010.

Insurers come off sidelines with warning.

The health insurance lobby, America’s Health Insurance Plans, came off the sidelines on Thursday to warn Senate leaders against repealing the Affordable Care Act’s mandate that most Americans have insurance without approving some mechanism to pressure people to maintain their coverage.

“We would oppose an approach that eliminates the individual coverage requirement, does not offer continuous coverage solutions, and does not include measures to immediately stabilize the individual market,” the group wrote.

AHIP played a major role in getting the Affordable Care Act passed in 2010 but has been reluctant to intervene in the fight over its repeal. On Wednesday, the Blue Cross Blue Shield Association, a narrower insurance lobby, weighed in with a similar warning.

Both groups were pulled into the fray by expectations that the Senate could end up voting in the early morning hours of Friday on a narrow bill that repeals a few important parts of the Affordable Care Act but leaves much of the law in place. Two of the pieces that would be repealed are the mandates that individuals have health insurance and that large employers cover their employees. The Senate had intended to repeal those mandates but create a new rule that anyone who allows coverage to lapse would have to wait six months before getting a new policy.