President Donald Trump is suing New York District Attorney Cyrus Vance and his own accounting firm, Mazars USA, in an attempt to block a subpoena for Trump’s tax returns.

His lawyers argued that any criminal investigation into a sitting president would be unconstitutional, writing that because presidents have such a powerful role in government, they cannot be subject to criminal investigations until they leave office.

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“The framers of our Constitution understood that state and local prosecutors would be tempted to criminally investigate the President to advance their own careers and to advance their political agendas,” reads the lawsuit. “And they likewise understood that having to defend against these actions would distract the President from his constitutional duties. That is why the Framers eliminated this possibility and assigned the task to supermajorities of Congress acting with the imprimatur of the nation as a whole.”

In memos issued during the Nixon and Clinton presidencies, the Justice Department decided that presidents have temporary immunity while in office, so federal prosecutors are barred from charging sitting presidents with federal crimes. However, there is no established precedent for immunity from state prosecutors.

On Sept. 16, Vance subpoenaed eight years of Trump’s personal and corporate tax returns from Mazars USA, prompting the lawsuit.

Trump promised to release his tax returns during the 2016 campaign but never did. Democrats believe the tax returns could show clear evidence of hush-money payments before the 2016 election.

Trump’s former lawyer, Michael Cohen, paid adult-film actress Stormy Daniels $130,000 during the election in exchange for silence about her alleged affair with the president. Vance’s office is investigating whether reimbursement from the Trump organization to Cohen for the hush-money payment violated New York state law.

Earlier this year, a House committee similarly subpoenaed eight years of accounting records from Mazars USA. Trump sued Mazars and the committee. Trump lost the suit, but has appealed the verdict.