Wilmington, Del.― A federal judge on Wednesday delayed accepting a plea deal for President Biden's son Hunter, saying the terms as written by prosecutors and defense lawyers may not be constitutional, but also signaling the agreement could be approved in the future.

The deal that had been struck in June began to unravel near the start of the three-hour hearing. U.S. District Judge Maryellen Noreika asked a series of questions that revealed a disagreement between federal prosecutors and Biden's lawyers over whether the agreement - in which he would plead guilty to two tax misdemeanors and likely avoid jail time - would protect him from the possibility of additional criminal charges.

The complications marked another twist in a case that has been clouded for years by questions about possible political bias, prosecutorial delay, and debate over whether Hunter Biden was being treated too harshly or too gently because of his father's status as a former vice president and, later, president.

While the judge pressed the prosecutors and defense attorneys to resolve the immunity issues, she also expressed concern that they had crafted a two-step plea deal in which some key features may not be reviewable by the court.

The sides had proposed that Biden would plead guilty to the tax charges in a fairly standard agreement that requires the judge's approval. Separately, they crafted a "diversion agreement" with Biden's attorneys in which the president's son would admit to wrongdoing in the gun case and agree to certain conditions, including not purchasing a firearm and not using drugs, to avoid actually being charged with unlawful possession of a firearm.

That type of agreement is not typically approved by a judge. But this particular diversion agreement referenced the proposed plea deal and prosecutors submitted it to the judge, creating a bifurcated deal in which the assurances Biden wanted - that he will not be pursued for other tax or foreign lobbying charges - were not part of the tax case, but part of the gun diversion agreement, lawyers said in court.

A provision of the gun diversion agreement said that if Biden failed to remain drug free and meet other conditions for the next two years, the judge would determine whether he had broken the terms of the deal and tell prosecutors they could revive the gun charge against him.

But Noreika questioned whether she could lawfully do that, given that she is not a party to the diversion agreement and judges generally are not responsible for pursuing criminal charges.

"I have concerns about the constitutionality of this provision, so I have concerns about the constitutionality of this agreement," she said in court. She told the two sides to work further on the issue. In the meantime, as the unresolved situation means the criminal charges are still active against Biden, he formally entered a not guilty plea.

Deals to plead guilty can sometimes fall apart under closer scrutiny from a federal judge, but even when that happens, the two sides often find a way eventually to resolve the issue and enter a deal acceptable to the court.

At the start of Wednesday's hearing, Biden said he was prepared to enter the plea. But then Noreika asked whether he would still do so if it was possible additional charges might be filed against him in the future.

When Biden answered no, the two sides were suddenly at odds.

"As far as I'm concerned, the plea agreement is null and void," Biden lawyer Chris Clark told the judge at one point.

A side discussion between the prosecutors and defense lawyers revealed the extent of the disagreement.

"I don't know what you're trying to accomplish by blowing this up," Clark told prosecutors. One of those prosecutors, Leo Wise, pointed to papers related to the case and said he was bound by the terms in them.

Clark shot back: "Then we misunderstood, we're ripping it up."

The probe was opened in 2018, during the Trump administration, and has been a favorite talking point for Republican critics of President Biden and his family. Republican politicians have repeatedly accused Hunter Biden of broad wrongdoing in his overseas business deals and, since his father was elected, predicted that the Biden administration would be reluctant to pursue the case.

Attorney General Merrick Garland has said repeatedly that politics would not interfere with the criminal investigation, noting that it was being led by Delaware U.S. Attorney David Weiss, a holdover from the Trump administration, and saying that Weiss had complete discretion over whether to bring charges.

Those assurances have come under intense scrutiny in recent weeks, however, since Republican lawmakers released interview transcripts from two whistleblowers from the Internal Revenue Service who were part of the investigation. They told the House Ways and Means Committee that the Justice Department slowed and stymied the probe, whittling away the most serious evidence of alleged tax crimes. And they alleged that Weiss had told the investigative team his charging options were limited.

Officials said Monday that Weiss, who has pushed back against the whistleblower's allegations, is willing to testify to Congress about the investigation in the fall.

Papers filed in federal court in Wilmington when the plea agreement was reached indicate that Hunter Biden had agreed to plead guilty to two misdemeanor tax charges of failure to pay in 2017 and 2018. A court document says that in both those years, Biden was a resident of D.C., received taxable income of more than $1.5 million and owed more than $100,000 in income tax that he did not pay on time.

Prosecutors planned to recommend a sentence of probation for those counts, according to people familiar with the negotiations who spoke on the condition of anonymity to describe elements of the case that are not yet public. Hunter Biden's representatives have previously said that he eventually paid the IRS what he owed.

A second court filing is about the charge of illegally possessing a weapon, which involves a handgun Biden purchased at a time when he was abusing drugs. In that case, the letter says, "the defendant has agreed to enter a Pretrial Diversion Agreement with respect to the firearm Information."

Diversion is an option typically applied to nonviolent offenders with substance abuse problems. Hunter Biden has written and spoken openly about being addicted to cocaine during the years in question. People close to him, speaking on the condition of anonymity to talk candidly about a sensitive issue, said he sees the potential conclusion of the criminal case against him through the lens of an addict, and was hoping to use the guilty plea to admit to past mistakes, make amends for them and move on.

In all, prosecutors were planning to recommend two years of probation and diversion conditions, the people familiar with the plea deal said. If Biden successfully met the conditions of the diversion program, the gun charge would be dismissed at the end of that period, these people said.

But even if the legal turmoil that has surrounded Hunter Biden for years is wrapped up, the political battle will continue in the form of congressional investigations, attacks via social media and attempts to shape public opinion as President Biden seeks reelection in 2024.

A day before the court hearing, the chairman of the Ways & Means committee, Rep. Jason T. Smith (R-Mo.), filed a court brief asking Noreika to consider the IRS agent's account before accepting Biden's plea deal. That filing set off a legal fracas Tuesday night, as the judge questioned whether Biden's lawyers had misled her court clerk in seeking to get Smith's filing removed from the public case file.

Biden's lawyers told the judge in a letter that they did not mislead court employees and the issue was a misunderstanding.

The whistleblower's testimony offered a host of new allegations, including a text message that Hunter Biden allegedly sent on July 30, 2017, that invoked his father - at that time a former vice president - as the younger Biden tried to get a business partner to fulfill some expected promise.

White House press secretary Karine Jean-Pierre, speaking at the start of a White House briefing on Wednesday that began just as proceedings were breaking up in Wilmington, attempted to keep a distance from the case.

"Hunter Biden is a private citizen and this was a personal matter for him," she said. "As we have said, the president, the first lady, they love their son and they support him as he continues to rebuild his life."

She emphasized that "this case was handled independently, as all of you know, by the Justice Department under the leadership of a prosecutor appointed by the former president, President Trump."

Wednesday's court appearance took place a few miles from where Hunter Biden grew up and went to high school - just blocks from the train station from which his father commuted to Washington for decades as a member of Congress, now named the Joseph R. Biden Jr. Railroad Station.

The courthouse is located in the J. Caleb Boggs Federal Building, named after the Republican incumbent senator whom a 29-year-old Joe Biden narrowly defeated to begin his national political career in 1972.

The president's son and his allies recently have shown an increasing willingness to challenge his adversaries, at times against the wishes of those close to the president, according to people with knowledge of those conversations, who also spoke on the condition of anonymity to discuss them.

The younger Biden and his attorneys have filed a countersuit against the computer repair shop owner who said Biden dropped off his laptop and never claimed it. They have asked for congressional ethics actions to be taken against Rep. Marjorie Taylor Greene (R-Ga.), who last week displayed sexually explicit images of Hunter Biden during a congressional hearing.

And they have waged battles with Rep. James Comer (R-Ky.), chairman of the House Oversight Committee.

Separately, Hunter Biden last month settled a long-standing child support dispute with an Arkansas woman who is the mother of one of his children.

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Barrett and Viser reported from Washington. Ajay Patel and Daniel Steenkamer in Wilmington, Del. contributed to this report.