Washington (CNN) The federal judge overseeing the case against former national security adviser Michael Flynn set a December sentencing date, despite concerns from Flynn's lawyers, who mounted a fierce assault in court against the special counsel team that charged Flynn two years ago.

Prosecutors also subtly opened the door to asking the judge to send Flynn to prison. They previously recommended only probation, but that was largely hinged upon Flynn's role as a model cooperator. Since then, Flynn's relationship with the Justice Department has all but collapsed. Asked by the judge during Tuesday's court hearing, prosecutor Brandon Van Grack declined to say that the earlier recommendation still stands.

Judge Emmet Sullivan of the DC District Court said Tuesday he wanted to set a "tentative" date for sentencing on December 18, though there are still a handful of issues that need to be settled first.

Flynn was President Donald Trump's first national security adviser in the White House but was fired after only a few weeks on as his exposure in the widening Russia investigation became untenable.

Tuesday's hearing came at virtually the same time Trump announced he had fired John Bolton as national security adviser.

The new sentencing date is exactly one year after Flynn's previous sentencing hearing, which was abruptly delayed to give Flynn more time to assist federal investigators. The case has seen a seismic shift over the past year. Flynn transformed from a repentant government cooperator to a leading Justice Department antagonist, whose new team of lawyers are arguing that he was only charged because of "egregious government misconduct."

Defense attorney Sidney Powell relied heavily on unproven conspiracy theories to make those arguments in court Tuesday, and was met with skeptical questions from Sullivan, who pointed out that some of her chief complaints were not directly related to the crime that Flynn admitted committing.

Powell claimed that the Justice Department was withholding critical evidence that would destroy "their whole notion that (Flynn) was any sort of agent of any foreign country." But Sullivan pointed out that Flynn pleaded guilty to lying to the FBI about his Russian contacts -- not with being a Russian agent.

Pressed by Sullivan, Powell said she wasn't angling to have Flynn withdraw his guilty plea. Instead, she wants Sullivan to throw out the charges because of the supposed government misconduct.

"The entire prosecution should be dismissed," Powell said.

The strategy puts Flynn in a somewhat awkward position because he has previously affirmed under oath -- twice -- that he was genuinely guilty of lying to the FBI. And during his planned sentencing hearing year, Flynn even told the judge that he had no qualms about how the FBI handled his interview, which is a key sticking point in some of the arguments that his new lawyers are trying to make.

"The court afforded Mr. Flynn and his attorneys to seek the withdrawal of that plea if they wanted to," Sullivan said, referring to an aborted sentencing hearing in December 2018.

Prosecutors have maintained that they followed all the rules and gave Flynn's lawyers all the information they needed to appropriately defend him, including 22,000 pages of documents.

Tuesday's hearing was the latest development in the slow deterioration of Flynn's cooperation deal with the Justice Department. In a filing last month, Flynn's lawyers accused special counsel Robert Mueller's team of suppressing key evidence and manipulating press coverage to corruptly "extort" a guilty plea.

The filing featured 19 pages of conspiracy-tinged theories that have received significant attention from right-wing news outlets but have largely stayed out of official court proceedings in Mueller-related cases. This includes thinly supported allegations that the FBI and Justice Department were "infected" by sinister officials who oversaw an "illicit operation" to stop Trump from becoming president.

Powell raised some of those theories in court, and name-dropped figures in the Mueller investigation who routinely pop-up in conservative outlets. She said former FBI Director James Comey was a liar, and referred to the anti-Trump text messages from former FBI officials Lisa Page and Peter Strzok. Trump has said those former officials were part of a "deep state" that has tried to undermine him.