Meet the prosecutor who could be Robert Mueller's Supreme Court closer

Special counsel Robert Mueller built a team of more than a dozen prosecutors to investigate Russian efforts to interfere in the 2016 election, but experts say one member might best be considered "the closer": Michael Dreeben.

"I don't have many insights into his legal approach except to say there is no human being, on the planet, with more knowledge about federal criminal law than Michael Dreeben, and no one with more expertise than him," said Leah Litman, a constitutional law professor at University of California at Irvine.

Dreeben is one of the government's most venerated and tested Supreme Court specialists. His career at the Office of the Solicitor General, the lawyers who represent the federal government before the high court, spans nearly three decades. He has argued more than 100 cases before the U.S. Supreme Court, making him "only the second person to reach that rare milestone this century," declared Chief Justice John Roberts during the court hearing that marked the occasion.

And he could prove a powerful asset in a case that could eventually pit the Justice Department against the president of the United States.

One of his particular areas of expertise has been in search and seizure law, explained Matt Olsen, a former Deputy Assistant Attorney General and now an ABC News contributor. Dreeben has recently argued, for example, that federal prosecutors should have access to digital data stored outside the United States and that the government's collection of cell-tower records does not violate constitutional protections against unreasonable searches.

In fact, it was that topic that brought Dreeben into court this week. He made an early appearance for the Mueller team in federal court in Washington to argue the government's position that former Donald Trump campaign chairman Paul Manafort's criminal case should not be dismissed and that certain seized evidence should be admissible.

At the hearing, Dreeben sought to assure the judge that the special counsel is checked in his authority by both Deputy Attorney General Rod Rosenstein, who must approve actions that might be beyond Mueller's mandate, and by Congress, which exercises oversight of the Department of Justice.

Peter Carr, a apokesman for the special counsel's office, declined to comment for this report. While few know where the Mueller case will head next, legal analysts expect Dreeben to play a key role in ensuring that special counsel prosecutors stay on the right side of the Constitution.

"I can imagine Michael being responsible for a host of pressing constitutional issues that might arise, ranging from the indictability of a sitting president to the lawfulness of the use of the pardon power," said Paul Rosenzweig, a former Whitewater investigator and senior fellow at the conservative think tank R Street Institute.

Another potential constitutional brawl "would be a fight for a compelled interview if the president refused to sit down with Mueller," said Jonathan Turley, a constitutional law professor at George Washington University Law School, who noted that the law in this area remains unsettled.

Most prosecutors on the Mueller team have maintained a low profile, but there have been hints in recent weeks that some Trump supporters plan to challenge the integrity of the probe by highlighting the political leanings of some of the prosecutors on the case. Veteran appellate lawyers told ABC News they cannot imagine any critique of that nature sticking to Dreeben.

"I have no idea what his politics are, but I know he is as faithful to the Constitution and laws of the United States as anyone who has ever served in government. Period. He is the consummate public servant," said Neal Katyal, a former acting solicitor general under President Barack Obama.

Katyal said that Dreeben cares more about fairness than winning. He recalled an incident when Dreeben told him, "We won this case in the court of appeals, but we really should have lost it. So let's tell the Supreme Court to hear the case and rule against us."

Marty Lederman, a professor at Georgetown University Law Center who worked with Dreeben at the Justice Department for over a decade, said the veteran prosecutor is not swayed by politics.

"With him and others like him on the team you can be confident that the prosecution will go where it ought to," Lederman said.