In 2015, Republicans changed the budget rules in Congress so that official scorekeepers would be required to analyze the potential economic impact of major legislation when determining how it would affect federal revenues. But on Thursday, hours before they were set to vote on the largest tax cut Congress has considered in years, Senate Republicans opened an assault on that scorekeeper, the Joint Committee on Taxation, and its analysis, which showed the Senate plan would not, as lawmakers contended, pay for itself but would add $1 trillion to the federal budget deficit.