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.

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.

Majority Leader John Thune's comments are a sign of the growing pressure he faces to get Republican hold-outs on board.