Donald Trump has sued New York state Attorney General Letitia James, seeking to block her from continuing to investigate his company.
The former US president argues she has violated his constitutional rights by pursuing a politically motivated probe.
Ms James says the investigation “will continue undeterred”.
She opened a civil inquiry in 2019 into claims that before he took office Mr Trump had inflated the value of his assets to banks when seeking loans.
The lawsuit comes less that two weeks after Ms James indicated that Mr Trump could be questioned under oath next month as part of the inquiry.
Mr Trump is also suspected of understating his assets to lower his taxes.
Ms James’ office has been seeking documents on four Trump Organization properties in Manhattan, upstate New York, Chicago and Los Angeles.
The former president, who is a Republican, has described the inquiry as a witch hunt.
Throughout his presidency from 2017 to 2021, Mr Trump resolutely refused to reveal his tax returns, despite coming under great pressure to do so.
Civil cases usually have to do with injury to individuals or other private parties, including businesses; criminal law applies in cases where the damage is thought to affect society at large, including the state.