As Congress moves towards final consideration of the GOP tax plan, there’s a lot of grumbling about the fact that it will eliminate the State and Local Tax deduction (SALT) in favor of allowing taxpayers to deduct $10,000 of state or property taxes.

While that has left Members of Congress from California, New York and New Jersey– perhaps most prolifically, Rep. Lee Zeldin (R-NY)– fuming, it has also upset some of the biggest special interests in the nation’s capital, predictably including realtors and mayors, who, respectively, worry it will diminish demand for pricier houses and force cities with income taxes to slash them.

But arguably an even more powerful, and vocal, opponent of stripping the SALT deduction has emerged: The American Federation of Teachers.

The American Federation of Teachers is the smaller, and more aggressive of the two big national teachers unions (the National Education Association boasts more members than the AFT). The AFT is also headed by Randi Weingarten, the top nemesis of charter schools in the country, depicted as an antagonist in “Waiting for Superman,” and a bankable “get” for just about any lefty group campaigning on just about any lefty cause, whether it be net neutrality or gun control.

It’s no surprise, then, that the AFT under Weingarten’s tenure, would be speaking out against the GOP tax bill.

But Weingarten’s focus on SALT is less ideological, and more practical– and therefore, more laser-like.

State and local taxes are the primary source of funds for public sector labor unions, like the AFT, which function because of member dues– which in turn are funded out of union-workers salaries.

Because education still remains a fundamentally state and local matter (despite Weingarten and her allies’ attempts to fully federalize it), Weingarten is literally worried that the elimination of SALT could wipe out her union, claiming that 90 percent of public school funding comes from state and local taxes. Weingarten believes that the elimination of the deduction will indeed force states and localities to cut taxes, just as that provision of the GOP tax bill was designed to do.

As such, the AFT has joined the anti-SALT elimination Americans Against Double Taxation, of which the NEA is also a member, in signing a letter urging maintenance of the SALT deduction.

Yesterday, that coalition issued a statement pushing again on the SALT deduction point, which has been getting significant attention in Washington, D.C.

The big question that presents is, how effective will Weingarten’s argument be in forcing last-minute vote-switching.

On the one hand, few legislators from either party want to get tagged with setting up policy that deprives well-liked and trusted teachers of income, or local schools of basic funding.

On the other, Weingarten has spent her career making herself into perhaps the poster-child for everything the GOP, and many independents, hate about unions, especially those organizing public sector workers who function on the taxpayer dime.

If you’re a proponent of stripping out the SALT deduction, you could hardly ask for a better foe. Expect Republicans who vote for the bill but take heat from their Democratic opponents for it to invoke Weingarten and her union’s opposition to the bill as a reason for their vote, not for reconsideration of it.

The fact remains that whether thanks to “Waiting for Superman,” films like “Won’t Back Down,” pieces like the “Rubber Room,” many kids’ poor experiences with public education, and teachers unions that have forced parents to lose vacation time and sick leave with walkouts to protest figures like Wisconsin Gov. Scott Walker, the AFT in particular has less goodwill than Weingarten would like.

In all probability, the bill will pass and be signed into law by President Trump. If it does, we may find in two or three years that it has been the most consequential move in a long time to curb the power of teachers unions, and force reform of the U.S. education system.