Elizabeth MacDonough, the Senate Parliamentarian, has flagged a growing list of provisions that would be prohibited under the Byrd Rule.

The Byrd Rule, adopted in 1985, is a procedural constraint named after the late Senator Robert C. Byrd of West Virginia to prohibit “extraneous” provisions from being tacked onto reconciliation bills, which are fast-tracked budget packages that allow legislation to pass with a simple majority, bypassing the 60-vote filibuster threshold.

Gone are GOP-led efforts to curb environmental rules, attempts to restrict federal judges' powers and plans to bulk up immigration enforcement.