WASHINGTON >> Hunter Biden asked a judge on Wednesday to approve subpoenas for documents from Donald Trump and former Justice Department officials related to whether political pressure wrongly influenced the criminal case against 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 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 subpoenas 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.

The subpoenas would seek documents and other communications about the investigation, including its origins and charging decisions. Representatives for Trump and the three former top Justice Department officials did not immediately return email messages seeking comment.

Hunter Biden’s attorney Abbe Lowell argued the information was essential to his defense that the case is “possibly, a vindictive or selective prosecution arising from an unrelenting pressure campaign beginning in the last administration,” that violated his rights.

The subpoena request is before U.S. District Judge Maryellen Noreika, a Trump nominee whose questions about a proposed plea deal over the summer ended with the agreement imploding in July.

Hunter Biden had been expected to plead guilty to misdemeanor tax charges in an agreement that would have spared him prosecution on a gun count if he stayed out of trouble for two years.

It had been pilloried as a “sweetheart deal” by Trump and congressional Republicans investigating nearly every aspect of Hunter Biden’s business dealings and the Justice Department’s handling of the case.

Hunter Biden has taken a more aggressive legal approach in recent months, striking back with lawsuits against Republican Trump allies who have traded and passed around private data from a laptop that purportedly belonged to him.

No new tax charges have yet been filed, but the special counsel overseeing the case has indicated they are possible in Washington or in California, where Hunter Biden lives.