The federal judge presiding over Hunter Biden’s gun case in Delaware on Friday rejected Mr. Biden’s claim that he was being subjected to selective prosecution, saying it was “nonsensical” that the Biden Justice Department would target the president’s son.

Abbe Lowell, Hunter Biden’s lawyer, has filed a flurry of motions in the Delaware gun case and a separate indictment in California on tax charges, accusing the government of unfairly singling out his client at the instigation of Republicans and seeking to dismiss the charges. None of those challenges have been successful so far.

Judge Maryellen Noreika, who scuttled a plea deal reached between prosecutors and Mr. Biden last summer, said that Mr. Lowell failed to provide evidence that prosecutors had been motivated by animus against Hunter Biden.

The “defendant’s claim is effectively that his own father targeted him for being his son, a claim that is nonsensical under the facts here,” Judge Noreika wrote in her 25-page decision.

The judge also rejected Mr. Lowell’s claim that David C. Weiss, the special counsel and U.S. attorney in Delaware, had only decided to bring charges against Hunter Biden because of pressure from Republicans in Congress who claimed attempts to reach a plea agreement last year were a “sweetheart deal” intended to protect the Bidens.

“Regardless of whether congressional Republicans attempted to influence the executive branch, there is no evidence that they were successful in doing so,” she wrote.

A federal grand jury in Wilmington indicted Hunter Biden in September on charges that he lied about his drug use on an application for a Colt pistol in 2018.

In response to a question on the form about whether he was using drugs, Mr. Biden said he was not, an assertion that prosecutors concluded was false. Mr. Biden has publicly acknowledged his struggles with addiction to crack cocaine and alcohol and had been in and out of rehab around the time of the gun purchase.

If convicted, Mr. Biden could face up to 25 years in prison and $750,000 in fines. But nonviolent first-time offenders who have not been accused of using the weapon in another crime rarely get serious prison time for the charges.

The decision to file criminal charges against President Biden’s troubled son was an extraordinary step for the Justice Department and Mr. Weiss after the last-minute collapse of a deal that would have granted Hunter Biden broad immunity from future prosecution on gun and tax charges without serving prison time.

In December, a separate federal grand jury in Los Angeles charged the president’s son with a scheme to evade federal taxes on millions in income from foreign businesses.

Hunter Biden faces three counts each of evasion of a tax assessment, failure to file and pay taxes, and filing a false or fraudulent tax return, according to the 56-page indictment.

Both trials are scheduled to begin in June, although the schedules are subject to change.