President Donald Trump on Tuesday sued California over a law that requires candidates for presidential elections to release their income tax returns in order to be eligible to appear on the state’s primary ballot.

Trump’s attorney William Consovoy claimed in a lawsuit that California’s law adds an “unconstitutional qualification” to the set of requirements for a presidential campaign. Consovoy also argued the provision violates the Constitution’s First Amendment.

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On Tuesday, GOP voters — along with the Republican Party of California and the Republican National Committee — filed a similar lawsuit that accused the state’s new law of being a political scheme to strip Trump supporters of their voting rights.

Last month, California Democratic Gov. Gavin Newsom signed the state’s Presidential Tax Transparency and Accountability Act.

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“The United States Constitution grants states the authority to determine how their electors are chosen, and California is well within its constitutional right to include this requirement,” Newsom said in a statement.

Presidential elections are typically viewed as a federal matter. Thus, the state of California’s new requirement for presidential candidates will likely face pushback from people who say it violates political parties’ rights to choose their nominees.

Rick Hasen, a legal expert from the University of California, Irvine, told CNN the state’s new law is of “uncertain constitutionality.”

“It is unclear whether state legislative power contained in Article II of the (US) Constitution gives California the power to take this step,” said Hasen.

“If such a law is upheld as constitutional, we might see a race to the bottom whereby other states enact ballot access requirements, such as a requirement for candidates to produce a birth certificate, which could affect which candidates can run in which states,” he added.

In recent months, many Democratic lawmakers have sought to obtain Trump’s federal tax returns via subpoenas of accounting firms or through other means. Among the congressional committees seeking to acquire Trump’s taxes are the House Ways and Means Committee, the Senate Finance Committee and the Joint Committee on Taxation.  Aside from California, New York’s Democratic-led state legislature has also pursued the president’s state taxes via a law, and Trump has sued the state in response. New York is home to Trump’s many businesses and homes like Trump Tower.

Trump long claimed he couldn’t release his taxes because he is under audit, although the Internal Revenue Service has explicitly said an audit does not preclude anyone from disclosing his or her tax returns.