White House budget director Mick Mulvaney defended the Trump budget maneuver, saying that counting the tax plan as revenue-neutral "was the most reasonable option we had."

Treasury Secretary Steve Mnuchin pledged to be transparent about the Trump tax plan, eventually.

"We felt it was premature to put in any changes to the budget as a result of taxes since we are not far enough along to estimate what that impact will be," he said at an event sponsored by the Peter G. Peterson Foundation, which says it seeks long-term government fiscal solutions. Mnuchin said the cost of the tax cut won’t be "anything like $4 trillion or $5 trillion" and noted that the administration has proposed ending the state and local tax deduction for individuals.

Rosy Growth

The Trump budget is able to claim $2 trillion in additional revenue over the next decade because it forecasts real gross domestic product growth jumping to 3 percent by 2021 and staying at that level for years. Senate Budget Committee Republicans pointed out in their own analysis that this is above the 1.9 percent average growth that the Congressional Budget Office projects.

"An aging population and falling productivity make 3 percent growth very unlikely, as history and basic economics teaches us," Bell said.

Asked about criticism that the administration was cooking the books, Mulvaney said Tuesday that early Obama administration budgets projected rapid economic growth that never materialized.

In his 2012 budget request for example, President Barack Obama projected growth would jump from 2.7 percent in 2011 to 4.4 percent in 2013 and remain above 3 percent through 2016. Those numbers never materialized.

"Every presidential budget is aggressive and I don’t know that this is qualitatively different," Doug Holtz-Eakin, a former Republican-appointed Congressional Budget Office director, said of the Trump growth assumptions.

Even so, the nonpartisan Committee for a Responsible Federal Budget estimated that using the CBO’s growth assumptions, the budget would produce a $625 billion deficit in 2027. If the administration needs to rely on economic growth to make its tax plan pay for itself, then the deficit would be $1.1 trillion that year.