He's ripped Michael Flynn for lying to the FBI and issued an order halting the Justice Department harsh policies on asylum-seekers — and that was just this week.
Emmet Sullivan, the federal judge whose dressing down of Flynn added an extra layer of drama to the proceedings in Washington and whose asylum ruling may affect thousands of immigrants trying to come into the U.S., has a reputation for being tough, fair and unpredictable.
Sullivan "is a fiercely independent judge," Glenn Kirschner, a former longtime federal prosecutor in Washington, told NBC News.
"He's very polite, civil and cordial, and extremely demanding and exacting, particularly of the prosecution," he said. "He's somebody that loathes any hint of governmental misconduct. If we're doing something not to his liking, he'll let us know about it."
On her Fox News show Saturday night, TV personality Jeanine Pirro praised Sullivan as "a jurist unafraid of the swamp, a judge who has a track record of calling out prosecutorial misconduct, a man who does not tolerate injustice, or abuse of power."
She predicted he would throw out Flynn's guilty plea if he found the FBI had duped him into lying — but that's not what Sullivan found. After he got Flynn to acknowledge that he'd lied to investigators knowing full well it was a crime, he told Flynn, "This is a very serious offense" that could result in him seeing jail time.
"A high-ranking senior official of the government making false statements to the Federal Bureau of Investigation while in the White House," Sullivan said during the Tuesday hearing.
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Sullivan backtracked from some of his own comments during the hearing, including when he mistakenly accused Flynn of actively doing work for the Turkish government after becoming Trump's national security adviser. But the tough talk rattled Flynn, who agreed to put off his sentencing until he could show the judge further cooperation with federal investigators.
"General Flynn was governmental misconduct personified, and I think that's why Judge Sullivan was so vigorous in his condemnation of Flynn's crimes. He didn't want him to wiggle out of his responsibility for his actions," said Kirschner, now a legal analyst for NBC.
Less than 24 hours later, Sullivan made waves again, ruling Wednesday against a Justice Department policy that made it almost impossible for immigrants coming into the U.S. to seek asylum on the grounds of domestic or gang violence.
Because "it is the will of Congress — not the whims of the Executive —that determines the standard for expedited removal, the Court finds that those policies are unlawful," Sullivan wrote in his 107-page decision.
If the ruling holds up on appeal, it could allow thousands of immigrants to once again seek asylum in the U.S.
Sullivan showed off his temper earlier in that case, when he discovered the feds were deporting two of the plaintiffs in the lawsuit in August. He ordered the government "to turn that plane around either now or when it lands. Turn that plane around and bring those people back to the United States. It's outrageous."
The Justice Department complied, and brought the mother and child back from El Salvador.
"He's a stickler, and he will let both sides know if he believes they're doing anything that's out of line," Kirschner said.
Sullivan, 71, doesn't fit into easy political boxes. A native of Washington and a Howard University Law School grad, he was appointed to the bench in 1984 by President Ronald Reagan, a Republican. He was later promoted to a D.C. appeals court by another Republican president, George H.W. Bush, before being elevated to the federal bench by Democrat Bill Clinton in 1994.
The Clinton appointment didn't stop Sullivan from ripping into Hillary Clinton in 2015, when he was overseeing a Freedom of Information Act lawsuit involving her emails from when she served as secretary of state.
In another case, he threatened to find President Barack Obama's Internal Revenue Service Commissioner John Koskinen in contempt of court for failing to produce documents he'd ordered the IRS to turn over. He also reportedly threatened the Department of Justice lawyer representing the agency. "You're in a very difficult position, but you're walking out of court with your colleagues," the judge said, according to Fox News. "That might not always be the case,OK?"
Before this week, Sullivan was probably best known for his ruling throwing out the corruption conviction of Ted Stevens, a former Republican senator from Alaska. It was also the case that Sullivan has said had the biggest impact on how he views his job.
Stevens had been convicted of seven felony ethics violations in a weekslong trial before Sullivan in 2008. Sullivan dismissed the case after it emerged that prosecutors had withheld evidence that could have helped Stevens' defense. One of the prosecutors killed himself while being probed for his actions in the case.
Sullivan said he'd "never seen mishandling and misconduct" like he'd seen in the Stevens case, and has since fought to hold prosecutors personally accountable by the judicial system for withholding so-called Brady evidence.
"While the egregious intentional misconduct of the Stevens case thankfully does not happen every day, failure to comply with constitutionally required disclosures does occur," he wrote in a Wall Street Journal op-ed last year. "Judges have a responsibility to take action against unethical prosecutors and to exercise their supervisory authority to prevent Brady violations before they happen."
Dareh Gregorian is a politics reporter for NBC News.