Congressional Republicans are plainly eager to move on to tax reform after months of wrestling with the conflicting goals of eviscerating the Affordable Care Act and pleasing middle- and lower-income Republicans who depend on subsidized insurance. And with tax reform, the conventional wisdom goes, the sailing is bound to be smoother.

Indeed, the House leadership and the Trump administration broadly agree on key elements of a tax bill, including large rate reductions for individuals and businesses. However, they’ve said little about how they’d offset the cost of those rate cuts. A rare exception: their apparent determination to repeal the state and local tax deduction (aka SALT), now allowed under the individual income tax.

At first glance, repealing the SALT deduction seems an easy lift. Republicans control the White House and Congress, while the deduction mainly benefits residents of high-tax states that tilt Democratic. But a close look at the makeup of the House of Representatives suggests the picture is more complex.

Assuming no Democratic support, the House GOP leadership can’t pass a tax bill if more than 22 Republicans vote against it. But I counted the number of GOP lawmakers who represent states that would be most affected by a repeal of the SALT deduction, and there are more than enough Republicans from high-tax states to block the change.  

First, I looked at the ten states whose residents claimed the largest average amount of state and local taxes on their 2014 federal tax returns (the last year for which data are available). These ten (which include California, New York and New Jersey) send a total of 39 Republicans to the House. In other words, if the House leadership loses a bit more than half of those lawmakers, the SALT deduction survives.

Take New York, where residents claimed an average of $21,038 in state and local taxes on their federal returns in 2014. In the Empire State, which has nine GOP representatives, middle-income households (with adjusted gross incomes of $75,000-$100,000) that claimed the deduction took an average of about $10,000 in 2014. Thus, for households in the 25 percent bracket, losing the deduction would raise their federal income taxes by $2,500. It would not be hard for challengers (in both primaries and the general election) to attack incumbent lawmakers for “raising taxes on a typical New Yorker by $2,500.”

It’s the same in New Jersey, with five Republicans in the House. Here, too, the average deduction by a middle-income household was about $10,000. In California, with 14 Republican members of the House, a typical middle-income filer who took the deduction wrote off almost $8,000 in state and local taxes.

Wait, it gets worse. What if you counted the Republicans who represent states with the largest percentage of taxpayers who take the SALT deduction? New York and New Jersey are still on the top-10 list. Indeed, about 3.3 million federal returns filed by New Yorkers claimed a deduction for state and local taxes, about one-third of all returns filed in the state. California drops off the list (though barely). But here’s the rub: states with majority Republican delegations including Utah and Virginia join the list. In Utah, where all four members of Congress are Republicans, 35 percent of federal 1040s included the state and local deduction.

Perhaps, you say, it would be prudent for the Republican leadership to avoid the battle altogether. But not so fast: repealing the SALT deduction would increase revenue by $1.3 trillion over 10 years. And without that revenue, Congress would find it very difficult to significantly reduce tax rates without adding to the deficit.

The challenge would go from difficult to Hail Mary if Congress also failed to enact the House GOP leadership’s other revenue-raising priorities, including the border adjustment tax (already targeted by big retailers) or its proposal to repeal the deduction for corporate interest expenses. Combined, those two measures would generate an additional $2.3 trillion over a decade. Yet on June 13, House Ways & Means Committee Chair Kevin Brady (R-Tex.) said he would scale back both of those proposals.

Efforts to repeal the SALT deduction are hardly new. Tax reformers tried, and failed, to repeal it as part of the 1986 tax act. Congress dropped the idea after Democratic governors from high-tax states leaned on their Democratic House colleagues. Congress did eliminate the deduction for sales taxes, but only for a while. Under pressure from both Democrats and Republicans in states with a sales tax but no income tax (Alaska, Florida, Nevada, South Dakota, Texas, Washington and Wyoming), Congress restored the sales tax deduction in 2004. Today, nearly everyone who itemizes takes a deduction for state and local taxes.

Of course, any tax reform — or at least any that generated as much revenue as it gave back — would produce losers as well as winners. And GOP lawmakers may still vote for a bill that includes cuts to the state and local deduction, provided it offers sufficient goodies to powerful friends. But winning their support may prove a much tougher sell for Republicans than the free-lunch rhetoric of both the president and House GOP leaders suggests. Indeed, the SALT deduction is an object lesson in why it will be so tough for Congress to pass a tax reform that does not significantly add to the budget deficit.