Donald Trump. Joe Raedle/Getty Images Some Republicans aren't too happy with a key part President Donald Trump's tax plan.

The rough outline the administration released on Wednesday said all deductions would be eliminated except for those from charitable donations and mortgage payments.

One of the deductions that faces the chopping block is the state and local tax deduction (SALT). Americans can deduct the amount they pay in state and local taxes from their federal return, saving them money on that filing.

While eliminating the SALT deduction could save the federal government a massive amount of money, it also benefits many people in three states: New Jersey, New York, and California. According to the Committee for a Responsible Federal Budget, about a third of the benefit goes to those three states.

Those states have recently been three of the most Democratic-heavy during presidential elections. But it's Republicans in Congress from those three areas who have already started to push back about the provision's possible elimination, according to Bloomberg's Sahil Kapur.

"I think it’s very important that it remain in the code," Rep. Leonard Lance, of New Jersey, told Bloomberg.

Reps. Chris Collins and Tom Reed of New York also told Bloomberg that they were looking at the SALT deduction and were not sure that they supported that element of the Trump plan.

Even Republicans outside of these states could find political benefit in fighting against the elimination of the SALT deduction.

As noted by the New York Times' Nate Cohn, the SALT dedication is used more heavily by people in "vulnerable Republican" districts. And as the GOP vote share decreases in a congressional district, the value of SALT deductions per filer generally increases.

That means GOP lawmakers in the House who may find themselves in tough reelection fights have constituents very interested in keeping the deduction intact. Members Cohn pointed to include Rep. Barbara Comstock in Virginia, Rep. Erik Paulsen in Minnesota, and Rep. Randy Hultgren in Illinois, as well as some in the three blue-leaning states.

For instance, constituents in the district of Republican Rep. Peter Roksam of Illinois, the head of the House Ways and Means subcommittee on taxes, took the 10th highest value of SALT deductions per filer in Cohn's list and the third highest for a GOP member outside of California, New York, and New Jersey. When asked by Bloomberg about the deduction, Roksam replied, "We’re going to look at all that."

Conservative activist groups have long favored the elimination of the SALT deduction for some time.