(NBC News) - "A New Foundation for American Greatness" was built on quicksand, and the president's defenders are happy to keep shoveling it.

President Donald Trump's newly unveiled budget contains a massive accounting error that uses the same money twice for two different purposes. Based on its supersized projections of 3 percent GDP, the president's budget forecasts about $2 trillion in extra federal revenue growth over the next 10 years, which it then uses to pay for Trump's "biggest tax cut in history."

But then it also uses that very same $2 trillion to balance the budget.

White House budget director Mick Mulvaney didn't deny the math, saying it was done "on purpose," during a press briefing Tuesday.

"I'm aware of the criticisms and would simply come back and say there's other places where we were probably overly conservative in our accounting," he said. "We stand by the numbers."

Treasury Secretary Steve Mnuchin, when asked Tuesday about the fiscal fumble during an economics conference, replied, "This is a preliminary document that will be refined."

Others say the numbers just don't add up.

"Using the revenue from feedback from economic growth toward deficit reduction," wrote the non-partisan Committee for a Responsible Federal Budget, "is inconsistent with recent statements that economic growth would be used to help finance tax reform."

"Inconsistent" is one way of putting it.

Former Treasury Secretary Larry Summers put it another way, writing in his blog, "It appears to be the most egregious accounting error in a presidential budget in the nearly 40 years I have been tracking them."