The attorneys general of California, Connecticut, New Jersey, and New York are fighting a proposed Internal Revenue Service rule aimed at preventing a workaround to the new cap on federal tax breaks for state and local taxes.

It’s a contentious issue that divides along party lines following passage of last year’s tax overhaul with Republican-only support. Certain states with residents hit by the change, like New York, have proposed to circumvent the new cap by having residents donate to state-run institutions and claim a charitable tax credit that is fully deductible at the federal level. The IRS wants to prevent that via the new rules.

“The proposed rules would undermine state sovereignty by depriving state and local governments of the revenue necessary to sustain vital public services," the Democratic state officials wrote in a letter sent to the federal tax collector Thursday. "In addition, the proposed regulations would upset the status quo for the individuals, charities, and governments that have come to depend on existing programs."

The officials added that they believe the IRS rule wouldn’t be enforceable in practice, and would violate longstanding precedent with regards to charitable giving to state-backed causes. “The IRS should withdraw its proposal — as both the law and common sense demand," they wrote. "The IRS should not play politics; it should instead confirm its longstanding interpretation of federal law.”

The four Democratic-run states have advanced a charitable tax credit workarounds to the new limit on so-called SALT deductions. In Friday’s letter, they argue that blocking the ability of states to offer that tax credit would violate longstanding precedent and favor taxes paid to the federal government over those made to states, cities, and counties.

State officials signing the letter also argue that the IRS favors corporations over individuals in its rulemaking, because the IRS’s proposed rule would allow corporations to continue to deduct charitable deductions to states from their federal taxes, while individual taxpayers could not.

“[T]he IRS’s proposal ... would favor corporations over people, because corporations could still deduct charitable contributions that trigger SALT credits while individuals could not,” the attorneys general wrote in their letter sent Friday. “Had Congress wanted to create these arbitrary distinctions, it would have done so by statute.”

Last year’s tax law caps the amount of state and local taxes that can be deducted in payments to the federal government at $10,000, a net tax increase for some individuals in locations with higher income and property taxes. It did not substantially change the federal deduction on state and local taxes paid by companies.

The SALT cap issue became politically charged during debate over tax cuts and reform last year, as Democrats and some Republicans in Democratic-leaning states or areas, like New York, opposed the change. Democrats have raised the issue in several congressional races that could prove key to determining the majority in the House of Representatives following the midterm elections.