U.S. President Donald Trump and his allies in the Senate face a political free-for-all over passage of his sweeping tax-cut and spending legislation, which Republican congressional leaders hope to enact in coming days despite growing resistance from different party factions.

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.

While the two bills are similar on the major tax provisions, how they work out their differences in the coming weeks will determine how quickly they can get a final product over the finish line. President Donald Trump is pushing to have the legislation on his desk by July 4th.