In the fight over whether President Donald Trump is violating the Constitution’s ban on accepting gifts or payments from foreign states, U.S. District Judge Emmet Sullivan relied on evidence from legal historians and former government ethics officers to move Congressional Democrats’ lawsuit against the president over a major hurdle.

“I think as a general matter [Sullivan’s] opinion is a testament to how important amicus briefs can be to a case,” said Brianne Gorod, chief counsel to the Constitutional Accountability Center, who argued on behalf of the 200 Democratic lawmakers in the case Blumenthal v. Trump.

Sullivan, who’s served on the U.S. District Court for the District of Columbia since 1994, denied the U.S. Justice Department’s motion to dismiss the lawmakers’ claims that Trump, through his private business holdings, is allegedly violating the foreign emoluments clause in the Constitution. That clause, designed to protect against foreign corruption, requires federal officeholders to obtain the consent of Congress before accepting any benefits from foreign states.

In his 48-page opinion, Sullivan adopted the broad definition of “emoluments” sought by the challengers after finding the government’s narrower definition “unconvincing” and “inconsistent” with the “text, structure, adoption, historical interpretation, and purpose of the Clause, as well as Executive Branch practice.”

The lawmakers drew support from six amicus briefs, but two in particular—by legal historians and former government ethics officials—carried the day in the judge’s analysis of the definition of emoluments and the clause’s scope; the consequences of adopting a broad definition, and how the clause has been applied by the executive branch over time.

“The court appreciates the illuminating analysis provided by the amici,” Sullivan said in his ruling.

Melissa Maxman, managing partner of the Washington office of Cohen & Gresser, was counsel to the five historians who claimed the Justice Department “cherry-picks from two insignificant dictionaries and from the historical sources, despite a mountain of evidence to the contrary” for its narrow definition of emoluments.

They countered that an “investigation of English language dictionaries published from 1604 to 1806, of the influential writings of Blackstone, Pufendorf, and Adam Smith, and of the contemporary usage by the Founding generation in the constitutional debates and in their private writings all confirm a broad definition of the word.”

“When you’re working with legal historians, you kind of let them take the rope and run with it,” said Maxman, an antitrust litigator. “I don’t claim to be a legal historian. They really wrote it.”

Sullivan was persuaded that the “weight of the evidence” in “founding-era dictionaries and other contemporaneous sources” supported the position advocated by the historians and the lawmakers.

The historians’ brief also played a key role in another emoluments clause ruling by U.S. District Judge Peter Messitte of Maryland in the case District of Columbia and Maryland v. Trump. That case focuses on alleged violations of the foreign and domestic emoluments clauses related to Trump’s ownership of the Trump International Hotel in Washington.

A three-judge panel of the U.S. Court of Appeals for the Fourth Circuit heard arguments in March on the government’s appeal of Messitte’s ruling that the attorneys general of D.C. and Maryland have standing to sue Trump. No decision has issued yet.

Maxman said she was not surprised the brief played a prominent role in both cases. “I thought it showed both judges are looking to traditional legal sources to interpret the meaning of currently unused terms in U.S. legal jargon,” she said. “We got into the veins and leaves on the trees and came to the right answer.”

Sullivan said he also found convincing arguments by 11 former government ethics officers in a brief filed by Tejinder Singh, partner in Goldstein & Russell. He countered the government’s claims that a broad definition of emoluments would result in “absurd consequences.”

The ethics officials told Sullivan that the government applies a “totality-of-the-circumstances” approach to emoluments clause questions “with a bias in favor of breadth, and a keen eye to the anti-corruption purpose of the clause.”

The judge also looked to opinions by the Justice department’s office of legal counsel and the Comptroller General as sources for how “emolument” is defined and how the clause has been interpreted and applied by the executive branch. What he discovered, Sullivan said, “clearly supports” arguments made by the lawmakers and the former ethics officers rather than those by the president.

For courts confronting the emoluments clauses for the first time, there could be concerns that the clauses, as the government governed, are impractical to enforce or to determine the “metes and bounds” of the president’s obligations, said Goldstein & Russell’s Singh.

“We sought to reassure the court that was not true,” he said. “Government officials in every branch have been complying with these clauses for decades now with the assistance of their ethics officials. They can be practically applied; they’re not difficult to apply, and they’re important to good governance.”

Singh called “very heartening” to see the district court citing and quoting from the former ethics officers’ brief. “That’s unusual for an amicus, but it shows their expertise was of value to the court.”

The judge turned to two other amicus briefs in his analysis: seven separation of powers scholars by Katharine Mapes, partner in Spiegel & McDiarmid; 16 federal jurisdiction and constitutional law scholars by Corey Roush, partner in Akin Gump Strauss Hauer & Feld.

A third emoluments clause lawsuit, CREW v. Trump, was dismissed on standing grounds in December 2017 and is pending decision on the appeal in the U.S. Court of Appeals for the Second Circuit.

Sullivan’s ruling in Blumenthal v. Trump is posted below: