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

The reconciliation process lets Republicans push the bill through the Senate—where they have a 53-47 majority—on a simple majority vote. But the process comes with strings attached, and those limits are known as the Byrd Rule, for the late Sen. Robert Byrd (D., W.Va.).