WASHINGTON — The Senate parliamentarian has advised that a Medicaid provider tax overhaul central to President Donald Trump’s tax cut and spending bill does not adhere to the chamber’s procedural rules, delivering a crucial blow as Republicans rush to finish the package this week.

Guidance from the parliamentarian is rarely ignored and Republican leaders are now forced to consider difficult options. Republicans were counting on big cuts to Medicaid and other programs to offset trillions of dollars in Trump tax breaks, their top priority. Additionally, the parliamentarian, who is the Senate’s chief arbiter of its often complicated rules, advised against various GOP provisions barring certain immigrants from health care programs.

Republicans scrambled Thursday to respond, with some calling for challenging, or ever firing, the nonpartisan parliamentarian, who has been on the job since 2012. GOP leaders dismissed those views and instead worked to revise the various proposals.

“We have contingency plans,” said Majority Leader John Thune of South Dakota.

Friday’s expected votes appeared to be slipping, but Thune insisted that “we’re plowing forward.”

But Democrats, who are unified against the package as a tax giveaway for the wealthy at the expense of American safety net programs, said the procedural decisions would devastate the GOP package.

Sen. Ron Wyden, the top Democrat on the Senate Finance Committee, said the Republican proposals would have meant $250 billion less for the health care program, “massive Medicaid cuts that hurt kids, seniors, Americans with disabilities and working families.”

The outcome is a setback as Senate Republicans race toward a weekend session to pass the bill and send it back to the House for another vote before Trump’s Fourth of July deadline. Trump hosted House Speaker Mike Johnson and other GOP lawmakers in the East Room at the White House, joined by truck drivers, firefighters, tipped workers, ranchers and others that the administration says will benefit from the bill.

At its core, the big bill, which has passed the House and is now being revised in the Senate, includes $3.8 trillion in tax breaks that had been approved during Trump’s first term but will expire in December, imposing a tax hike if Congress fails to act. To help offset lost revenues, Republicans are relying on steep cuts to health care and food stamps, and imposing new fees on immigrants.

GOP leaders were already struggling to rally support for Medicaid changes that some senators said went too far and would have left millions without coverage. The nonpartisan Congressional Budget Office has said more than 10.9 million more people would not have health care under the House-passed bill; Senate Republicans were proposing deeper cuts.