The conservative DC legal group behind a challenge to Mueller probe The National Legal and Policy Center is backing a subpoena fight.

The latest legal challenge to special counsel Robert Mueller’s Russia investigation, now heading to the U.S. Court of Appeals, is being steered by a veteran Washington legal group that has a history of taking on Democrats and is bankrolled, in part, by longstanding Republican donors.

The National Legal and Policy Center is backing a subpoena fight launched by Andrew Miller, a former associate of Trump confidant and political provocateur Roger Stone, who has refused a demand from prosecutors to appear before a grand jury. He is objecting, the lawyers said, in order to mount a broad legal challenge to the legitimacy of the special counsel probe.

“The government wanted to hold [Miller] in contempt,” said Paul Kamenar, who was hired by the policy center to mount a constitutional case against the Mueller team. “In order to appeal Judge Howell’s decision challenging the constitutionality of the special counsel we have to have a contempt order in order to go to a Court of Appeals.”

Kamenar told ABC News he believes Miller’s constitutional battle could head all the way to the Supreme Court. He has argued that Mueller was not a constitutionally valid special counsel because he should be considered a “principal officer of the United States,” and therefore should have been appointed by the president. The Department of Justice has taken the position that he is an “inferior” officer who could be appointed by a deputy, in this case, Deputy Attorney General Rod Rosenstein.

A recent challenge to the constitutionality of the special counsel was rejected by federal district court judge in D.C. earlier this week.

On Monday, a federal judge denied a move by Concord Management Consulting LLC, who was charged in the special counsel's probe earlier this year, to have their indictment dismissed based on virtually the same argument being used by Kamenar in Miller's case.

"The Court will deny Concord’s motion," U.S. District Court Judge Dabney Friedrich wrote in the Memorandum Opinion filed in D.C. federal court on Monday. "The Special Counsel’s appointment complies with the Constitution’s Appointments Clause because (1) the Special Counsel is an 'inferior Officer'; and (2) Congress 'by Law vest[ed]' the Acting Attorney General with the power to make the appointment."

In a statement reacting to the decision, Kamenar told ABC News on Monday, "Judge Friedrich did not reach the issue only presented in our case, namely, that Mueller even if he is an inferior officer, should have been appointed by Attorney General Jeff Sessions instead of Rod Rosenstein as a so-called Acting AG. That issue is part of our appeal."

Miller’s objections will be the first taken up on appeal.

The National Legal and Policy Center is a non-profit 501(c)(3) foundation with a deep history of mounting legal challenges against left-leaning organizations and Democratic politicians. In 1993, the group successfully sued to open the meetings and records of Hillary Clinton's health care task force.

On its website, the group touts its role in investigations that led to allegations of wrongdoing against a series of Democratic politicians. Among them were ethics charges against former Rep. Charles Rangel, D-NY, who was censured in 2010 for ethics violations, and Sen. Robert Menendez, D-NY, who was indicted in 2015 on corruption charges but acquitted earlier this year.

More recently, the group targeted Rep. Maxine Waters, D-Calif., filing a complaint July 26 with the Federal Election Committee alleging she violated election laws for some of her mailings. Waters' office has not yet responded to a request for comment from ABC News.

The group has received sizeable donations from well-heeled conservative foundations, including the Ed Uihlein Family Foundation, run by Right-wing mega-donor Richard Uihlein.

Uihlein was a major donor to pro-Trump GOP Senate primary challengers in the 2018 midterm elections. An Illinois shipping-supplies magnate who avoids the limelight, he "has risen to become one of the most powerful — and disruptive — GOP donors in the country," the Washington Post wrote in April.

Another prominent conservative NLPC donor, the Sarah Scaife Foundation, has deep ties to the Trump administration.

The most recent available charity tax filings show the Uihlein foundation gave NLPC $50,000 annually between 2010 and 2016, and the Scaife foundation gave the group $100,000 a year between 2010 and 2014, raising their donation to $125,000 in 2015. Flaherty told ABC News that both foundations are continuing supporters of NLPC. Neither organization responded to ABC News' request for comment.

The group first became interested in challenging the Mueller probe in May, when Flaherty says he saw an op-ed arguing that Mueller was not constitutionally appointed.

“I thought it would be worth pursuing,” Flaherty told ABC News. “So I was thinking it'd be a good project for the National Legal and Policy Center."

Flaherty said he reached out to longtime Stone associate and former Trump campaign official Michael Caputo, who he knew had met with special counsel investigators earlier that month.

“I asked [Caputo] if he knew anybody who might want to serve in this role and he said ‘as a matter of fact, there's a guy named Andrew Miller; let me give him a call,’” Flaherty told ABC News. ”He called Andrew. Andrew had a positive response.”

Miller had served as an aide to Stone, who has described Miller as his “wingman” because he helped manage Stone’s schedule, media appearances and offered other assistance to him around the 2016 Republican National Convention. Miller identifies himself as a registered libertarian and has said he did not support Trump’s candidacy.

The special counsel’s interest in Miller is unclear, but investigators have made repeated efforts to bring him in front of a grand jury impaneled for the Russia probe.

Already, attorneys for Miller had tried unsuccessfully two times in July to quash a subpoena requiring Miller to testify before the special counsel's grand jury, arguing that Mueller lacked the constitutional authority to issue it.

Last Friday, U.S. District Judge Beryl Howell held Miller in contempt of court for refusing to comply with the subpoena. Miller’s lawyer said they sought the contempt citation because it allows them to mount an appeal where they can advance the constitutional challenge. Now, the argument can be heard by the U.S. Court of Appeals.

“If we win in the Court of Appeals, of course, the government will go to the Supreme Court,” Kamenar told ABC News. “If we lose then we will petition the Supreme Court.”

Kamenar said that depending on how fast the Court of Appeals takes, the case could be brought to the Supreme Court by either party as soon as late this fall, or early next year.

The strategy is not without risk, according to Mitchell Epner, a former federal prosecutor.

“If Andrew Miller were to lose his appeal, and be denied review by the Supreme Court, and still persist in refusing to testify, then he likely would be subjected to penalties by the Court,” Epner said.

Penalties could range from fines to imprisonment until he agrees to testify – or the grand jury term expires.

“Some judges have imposed a $1 fine, which doubles daily. Within 10 days, that number becomes big enough to bankrupt virtually anyone."