President Donald Trump called for an end to "massive tax breaks" for the NFL over Twitter on Tuesday. But the biggest loophole of all for professional football could be in the president's own tax plan.

Trump appears to have been referring to the National Football League's former status as a tax-exempt nonprofit, similar to a business trade association. But the league gave up that position in 2015 after NFL Commissioner Roger Goodell said the organization had been "mischaracterized repeatedly."

"As a result, the committees decided to eliminate this distraction," Goodell wrote in a letter to team owners at the time.

In fact, the league's nonprofit status provided it very little tax benefit. One government estimate pegged the number at about $109 million over a decade — and that included not only the NFL, but also the National Hockey League, the PGA Tour and other professional sports leagues. For comparison, the NFL is expected to bring in $14 billion in revenue this year.

Much of that money gets distributed back to the teams — and that's where the potential for a giant loophole occurs. Almost none of NFL's teams are public corporations but so-called pass-through businesses. Under the tax framework negotiated by the White House and Republican leadership, owners and investors of pass-through businesses would see their tax rates fall from a maximum of 39.6 percent to just 25 percent.

The Tax Policy Center estimates the measure would result in $770 billion in lost tax revenue over a decade.

"This pass-through loophole is several orders of magnitude bigger than other issues," said Seth Hanlon, a senior fellow at the Center for American Progress, a left-leaning think tank. "It's just an enormous windfall."

A similar but much more dramatic shift has taken place in Kansas, where the state had cut the rate on pass-through businesses all the way to zero. That prompted Bill Self, the men's basketball head coach at the University of Kansas, to incorporate as a pass-through business to shelter $2.75 million in income from taxation, according to local news reports. Self was pilloried for the move, and the Kansas state legislature voted to begin raising tax rates this summer.

Republicans say they intend to write regulations to prevent wealthy households from taking advantage of the lower rate for pass-throughs. Treasury Secretary Steven Mnuchin said at CNBC's Delivering Alpha conference that some businesses, such as accounting firms, would not be allowed to take that benefit.

But it's still unclear if pro sports teams would also be barred. On that point, Trump's tweet was silent.

Trump tweet

Correction: President Donald Trump threatened to end the NFL's "massive tax breaks" on Twitter. A headline on an earlier version of this article erroneously cited "massive tax cuts."