WASHINGTON — President Trump slapped a lawsuit on Manhattan District Attorney Cyrus Vance Thursday, who last month subpoenaed eight years of Trump’s tax returns.

The move marks yet another front in Trump’s ever-widening legal battle to keep his finances, and specifically his tax returns, secret.

But this latest fight is about more than Trump’s taxes: In sweeping language, Trump claims immunity from any kind of criminal investigation whatsoever. The Constitution, his lawyers argue, gives a sitting president way too much responsibility to let him to be probed by prosecutors, of any stripe, while in office.

A sitting president cannot “be investigated, indicted or otherwise subjected to the criminal process,” the complaint says.

If ultimately successful, Trump’s claim would mark a massive power-grab, extending beyond the federal policy against criminally charging a sitting president. The assertion would give Trump, his companies, or even his old accounting firm a free pass from even having to answer questions from any nosy prosecutor so long as he’s in office.

A Department of Justice policy bars the indictment of a sitting president on federal charges. But the Supreme Court has never weighed in on whether a president can be indicted, let alone investigated, by a state official, like Manhattan’s Cy Vance.

The argument appears remarkably vast and looks unlikely to prevail in court, said Paul Rosenzweig, a former member of Ken Starr’s investigation into former President Bill Clinton.

“It cannot be the case that, as a matter of law, everybody associated with the president is immune from criminal investigation while he’s the president. But taken at its fullest, that’s what the president’s argument is,” Rosenzweig told VICE News.

Specifically, Trump’s new lawsuit aims to block Vance’s subpoena for eight years worth of tax returns from Trump’s longtime accounting firm, Mazars USA, on the grounds that he can’t be investigated to begin with.

“Because the Mazars subpoena attempts to criminally investigate a sitting President, it is unconstitutional,” Trump’s complaint reads, before going on to accuse New York State officials of waging a “campaign to harass the president.”

Vance has subpoenaed both The Trump Organization and Mazars for a vast pile of records as part of an investigation into hush-money payments made during the 2016 campaign to women who claim they slept with Trump.

The subpoenas seek documents and communications related to:

Payments made to Karen McDougal and Stormy Daniels, who say they had affairs with Trump

Trump’s son Don Jr.

Trump Org CFO Allen Weisselberg

Trump’s former attorney, Michael Cohen

Trump’s former spokeswoman, Hope Hicks

The National Enquirer, which bought Karen McDougal’s story but never ran it

And more.

It remains to be seen what a district-level judge will ultimately think of Trump’s claim, which may ultimately just serve to slow down the subpoena past the 2020 election.

“I think it’s clear that the president is engaged in an effort to conceal and delay,” said Rosenzweig, now a senior fellow at the nonprofit R Street Institute. “It’s hard to say whether that’s because there’s something bad there, or because his entire life indicates a general instinct to conceal and delay.”

But Trump seems to hoping the Supreme Court will weigh on his side, after he helped cement the high court’s conservative majority with the appointments of Brett Kavanaugh and Neil Gorsuch.