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.

Who is Senate parliamentarian Elizabeth Macdonough that ruled against Medicaid in Trump's 'big beautiful bill' Here's what to know about the Byrd rule

The Senate slogged through a tense overnight session that has now dragged into Tuesday morning, with Republican leaders searching for ways to secure support for President Donald Trump’s big bill of tax breaks and spending cuts while fending off proposed amendments,