Simmering beneath the discussions are a set of arcane rules that dictate how the Senate and House must account for policy changes that touch every corner of the economy. Optimistic growth assumptions are important because Republicans are planning to use a procedural tool called budget reconciliation to pass a tax bill through the Senate with a simple majority. The drawback of this expedited path is that if official projections from the Joint Committee on Taxation find that the plan will add to the deficit after a decade, the changes made to the tax code have to expire after that period of time.

Budget scoring can be influenced by a variety of factors, including economic forecasts and assumptions about future tax and spending levels. For instance, a model that assumes higher economic growth will project lower deficits since stronger growth tends to raise taxable income and reduce spending on low-income programs. The Office of Management and Budget and Congressional Budget Office both estimate that each 0.1 percentage point increase in annual economic growth reduces deficits by roughly $300 billion over the coming decade. Future deficit levels can also go up or down depending on the assumptions economists make about future tax and spending levels.

Republicans fret that the Joint Committee on Taxation and the Congressional Budget Office will underestimate the amount of economic growth from their tax cuts and plans to simplify the tax code. The Trump administration says its policies will get the economy growing at a rate of 3 percent, which they say will generate more than enough revenue to pay for the tax cuts.

“Any plan we put forward we believe should be paid for with economic growth,” Treasury secretary Steven Mnuchin said at a Senate hearing in May. “I am concerned as to whether some of the models will attribute enough growth in dynamic scoring, but when we present the details, we will present how we think it should be paid for.”

Yet economists also have concerns about that 3 percent growth number. The Federal Reserve this week said the economy is continuing to grow at a moderate pace and signaled it will likely raise interest rates in December. In March, the Fed estimated that the maximum sustainable pace of economic growth is around 1.8 percent and the Congressional Budget Office puts the ceiling at 1.9 percent. While low by historical standards, economic growth is determined by expansion of the work force and improvement in the amount that workers can produce, both of which have slowed in recent years as a result of an aging work force and lower productivity.

Meanwhile, veterans of past budget fights say while economic growth can alleviate the deficit hit from a $1.5 trillion tax cut it cannot offset it entirely.

“If it is a well-designed tax policy, it will partially offset the cost,” said Douglas Holtz-Eakin, a conservative economist who served in the George W. Bush administration and advised John McCain’s 2008 presidential campaign. But, he added, “there’s no evidence anywhere that a tax cut of that magnitude, regardless of composition, will offset” the entire $1.5 trillion.