WASHINGTON (AP) — Hunter Biden asked a judge on Wednesday to approve subpoenas for documents from Donald Trump and former Justice Department officials, claiming the former president applied political pressure to a criminal investigation of him.

Biden's attorneys allege there were “certain instances that appear to suggest incessant, improper, and partisan pressure applied” by Trump to his then-Attorney General William Barr and two top deputies, Jeffrey Rosen and Richard Donoghue.

While charges against President Joe Biden's son were not brought until this year, the investigation into into his taxes and a gun purchase began in 2018, while Trump, a Republican, was still president. The court filing cites public comments made by Trump, information from the House panel that investigated the Jan. 6, 2021, attack on the U.S. Capitol and details from Barr's book in which he described bubbling tension after Trump tried to pressure him over the status of the Hunter Biden probe.

The push for a subpoena comes as defense attorneys fight the federal firearms case filed against Hunter Biden, who is accused of breaking laws against drug users having guns. He has pleaded not guilty, and the case is on a track toward a possible trial in 2024 while his father, a Democrat who defeated Trump in 2020, is campaigning for reelection.

Hunter Biden had been expected to plead guilty to misdemeanor tax charges over the summer in an agreement with prosecutors that spared him prosecution on the gun count if he stayed out of trouble for two years. But that agreement imploded after a judge raised questions about it during a July hearing.

The special counsel overseeing the case has indicated that new tax charges are possible in Washington or in California, where Hunter Biden lives.

A representative for Trump did not immediately return an email message seeking comment.