Sen. Mike Lee, R-Utah, and Sen. Sheldon Whitehouse, D-R.I., two of the most vocal members of the Senate Judiciary Committee, clashed Thursday over comments Whitehouse made calling one of the judges now reviewing proceedings in the Michael Flynn case a "cartoon of a fake judge."

The dispute followed a request from Flynn's lawyers earlier this week for an emergency writ of mandamus -- an order from a higher court telling a lower official how to do their job -- from the D.C. Circuit Court that would order District Court Judge Emmet Sullivan to let the Department of Justice (DOJ) drop its prosecution of Flynn, overturn a previous order from Sullivan allowing outside parties to weigh in on the case and take Sullivan off of it.

A three-judge panel for the circuit court Thursday directed Sullivan to respond to Flynn's petition, a sign that it is taking the request seriously, provoking Whitehouse to slam one of the Trump-appointed judges in comments posted to Twitter.

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"Where you see [D.C. Circuit Court Judge] Neomi Rao, you can expect a lot of Trumpy dirt to follow," Whitehouse said. "She’s a cartoon of a fake judge. Watch this space."

Lee fired back, criticizing Whitehouse for an "ad hominem" attack on the sitting judge who Whitehouse has previously said has an "ideology ... birthed in the petri dish of the Republican corporate donor class."

"Not cool, Sheldon," said the senator, who has been on President Trump's Supreme Court shortlist. "Judge Rao is a gifted, hard-working legal scholar and jurist. I know you disagree with her here—and I strongly disagree with you on that point—but is it ever appropriate to call a sitting jurist 'a cartoon of a fake judge'? I can’t think of a good reason. Ever."

Lee continued: "I disagree with federal judges all the time. I even express my disagreement. But what good comes from this kind of an ad hominem attack on an individual jurist, using words like 'cartoon' and 'fake'?"

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Sen. Ted Cruz, R-Texas, also on the Senate Judiciary Committee, called Whitehouse's comments "shameful."

"Senate Dems, tired of threatening Supreme Court Justices, turn to the court of appeals for their partisan campaign of judicial intimidation," Cruz tweeted.

Carrie Severino, the president of the conservative Judicial Crisis Network, slammed Whitehouse for the attack on Rao, harkening back to an odd line of questioning about "boofing" Whitehouse pursued during Supreme Court Justice Brett Kavanaugh's confirmation hearings.

"This is the guy who introduced 'boofing' into the national vocabulary," Severino wrote. "He has threatened the Supreme Court in an amicus brief. And now personally attacking a federal judge?"

She added: "It’s a pattern of intimidation of judges. Schumer threatened two justices. Sheldon threatens to pack the court and attacks another judge. The Schumer-Sheldon assault on the judiciary is unprecedented, reprehensible, and unbecoming of a U.S. Senator and a member of the bar."

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Rao was confirmed to her seat in March of last year by a 53-46 vote in the Senate. She was sharply criticized by liberals largely for her writings as a college student, saying it was insensitive to sexual assault survivors, among other things. Rao was previously a staffer in the Office of Management and Budget's Office of Information and Regulatory Affairs under the Trump administration.

Her nomination was, however, praised by conservative court watchers and Senate Majority Leader Mitch McConnell, R-Ky.

"I’m glad @POTUS has nominated Neomi Rao to the DC Circuit Court Of Appeals," McConnell said in a tweet. "She has the top rating from the American Bar Association, and I’m confident she will be a fair and impartial judge on this important court."

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The DOJ's move to let Flynn, who pleaded guilty to lying to the FBI before moving to withdraw the guilty plea, off the hook was highly controversial. Critics said the Justice Department appeared to be making the move because Flynn is an ally of President Trump's -- he was the president's first national security advisor in 2017. But Attorney General William Barr defended the decision in an interview with CBS' Catherine Herridge.

"[W]e feel really that a crime cannot be established here because there was not, in our view, a legitimate investigation going on," Barr said. "They did not have a basis for a counterintelligence investigation against Flynn at that stage, based on a perfectly legitimate and appropriate call he made as a member of the transition."

Some court watchers have pointed out that Flynn appears to have drawn a disproportionately conservative panel from the D.C. Circuit Court, which overall has more Democrat-appointed judges than Republican-appointed judges.

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In addition to Rao, Flynn's request for a writ of mandamus is being overseen by Judge Karen LeCraft Henderson, a George H.W. Bush appointee, and Judge Robert L. Wilkins, who was appointed by former President Barack Obama.

"Fortunately, @GenFlynn⁩ drew a good panel of judges on the DC Circuit to hear his petition for writ of mandamus," Mike Davis, the founder of the Article III Project, a group dedicated to boosting Trump judicial nominees, said in a tweet. "The adults on this federal appellate court will likely put an end to the political nightmare and embarrassment unnecessarily caused by Judge Sullivan’s antics."

Sullivan has 10 days to respond to Flynn's petition to the circuit court, a time period in which the DOJ was also invited to weigh in.