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.