To the extent Douglas N. Letter caught a break, it came down to this: Two of his cases were being heard on the same floor of the same Washington courthouse on the same afternoon.

In one room, Letter tried to persuade a judge to force President Trump’s former White House counsel Donald McGahn to testify before Congress. During a short break, Letter hustled down the hallway to a second courtroom. There, his colleague insisted that former national security adviser John Bolton’s deputy must comply with a House subpoena to testify in the impeachment inquiry.

That fall day has come to represent a typical schedule for Letter, the genial, self-deprecating lead lawyer for House Speaker Nancy Pelosi (D-Calif.). After a 40-year career at the Justice Department, defending policies of presidential administrations from both parties, Letter now speaks for the speaker in courtrooms throughout the country while advising House leaders on impeachment.

With his small team of lawyers, Letter is locked in pitched battles with the Trump administration, which has vowed to fight all congressional subpoenas for documents and testimony — and resisted cooperating with House impeachment proceedings. As general counsel to the House, Letter has a hand in an outsize number of fast-moving legal fights between Congress and the president.

Letter is slated to represent the House at the Supreme Court, which will review two rare separation-of-powers cases over disclosure of Trump’s tax and financial records in March.

And in back-to-back hearings Jan. 3 at the federal appeals court in Washington, Letter will explain why the judges should give the House access to secret evidence from special counsel Robert S. Mueller III’s Russia investigation. His colleague, Megan Barbero, will then ask the court to uphold a ruling forcing McGahn to appear before a House committee despite White House efforts to block his testimony.

“Because of the stonewalling by the Trump administration and the insistence on going to court on every kind of thing, it’s very demanding with multiple opponents and multiple cases,” said Irv Nathan, who held Letter’s position the first time Pelosi was speaker, starting in 2007. “The job is on steroids.”

Justice Department lawyers counter in court filings that forcing key presidential advisers to answer Congress’s questions would be a “radical distortion of the balance of powers.” Grand jury material, they say, should be off-limits to Congress even in an impeachment proceeding.

The general counsel’s quarters are jammed into a second-floor suite of offices in the Cannon House Office Building, so cramped that one attorney works out of a closet. The tight space for nine lawyers and staff belies the lasting impact their cases will have on the structure of the nation’s government — the scope and limits of presidential and congressional power and how the branches interact.

“Even when he’s got a crushing workload on him and other people would snap, Doug’s reaction is to recognize what an incredible position he’s in,” said former Justice Department official David O’Neil, who has helped Letter’s team challenge the administration’s plan to add a citizenship question to the 2020 Census. “He inspires people to put in their best work and to pull all-nighter after all-nighter because the work matters.”

Letter, 66, sleeps only four hours on a typical night, according to friends and former colleagues. When he’s not in court or crafting legal filings, he briefs Democrats on strategy and advises House leaders on impeachment. He talks frequently with Pelosi in meetings, by phone or in texts, with messages that also touch on a shared affinity for sports teams from the Bay Area, where Letter grew up.

The House lawyers typically travel to hearings as a pack, entering the courthouse with Letter in his trademark baseball cap. While arguing against Trump’s border wall funding plan at the U.S. Court of Appeals for the 9th Circuit in San Francisco, he wore a 49ers cap. In Washington, it’s the Nationals or Capitals.

Letter, who was not authorized to comment for this article, was born in the District but spent his childhood in Palo Alto, Calif., where government work was a family tradition. His father, who served in World War II, ran the National Labor Relations Board office in San Francisco. Letter graduated from Columbia University and returned to California for law school at Berkeley.

A workaholic even by Washington standards, Letter holds to his sleep schedule to make sure he has time for long-distance bike rides, tennis and family, according to friends. They count on his Sunday evening emails asking who’s in for weekly basketball games at a Montgomery County, Md., middle school that Letter has organized for 35 years. Letter is the informal commissioner of the group, made up mostly of government workers who choose not to pay extra to air-condition the gym.

“Lawyers in this town can get competitive; we believe in our own version of the rules,” said Joshua Geltzer, a former Justice Department lawyer who worked with Letter during his stint last year at Georgetown’s Institute for Constitutional Advocacy and Protection. “If people get a little too combative or revert to lawyer mode, he’ll calm people down, tell us to take it easy.”

Letter has tapped his network of former government lawyers, including Geltzer and his Georgetown colleagues, to help handle the 15 pending lawsuits involving the House. The lawyers are working for free, according to a Pelosi aide.

When Letter retired from the Justice Department in February 2018, he was head of the civil appellate division with 65 attorneys. He had done stints in the White House Counsel’s Office during the Clinton administration, as terrorism litigation counsel defending post-9/11 policies for George W. Bush and as senior counsel to Barack Obama’s attorney general Eric Holder.

Ben Wizner, an American Civil Liberties Union attorney, argued opposite Letter in national security matters, including in a federal lawsuit against a CIA contractor for its alleged role in transporting detainees to secret prisons, where they were tortured. Letter, representing the government under Bush and Obama, narrowly prevailed in persuading a full panel of the 9th Circuit to dismiss the lawsuit because of secrecy concerns.

Wizner, who likes Letter personally, said his eyebrow raised when he learned of Letter’s new role.

“There is some irony in his being tagged to push back against executive branch overreach,” Wizner said. “As a government lawyer, he defended extravagant claims of executive prerogative, including that the CIA could designate its own illegal activity a state secret and thereby avoid any kind of accountability.”

But John Lawrence, Pelosi’s former chief of staff, sees Letter’s experience representing both Democratic and Republican administrations and his insights into high-level legal strategy as a plus.

“It never hurts to have somebody around who knows the other team’s signals or at least understands how they think,” said Lawrence, who recommended Letter for the job.

Chief Justice John G. Roberts Jr. and Justice Neil M. Gorsuch, both conservatives, attended Letter’s farewell party at the Justice Department, Lawrence noted.

“When he walks into court, the judges know he’s not there as a flack for the Democrats, but as a serious litigator,” Lawrence said.

Letter revels in bantering with the judges and is at ease in nerve-racking court sessions of hypothetical, rapid-fire questioning, said Matt Collette, who was Letter’s deputy at the Justice Department.

During a hearing in November over access to Mueller’s secret grand jury material, Letter pointed out that the Justice Department lawyer had repeatedly referred to the department as “the government.”

“The House of Representatives is the first branch,” Letter said. “We’re both the government, but we’re the first branch.”

“You’re part of the first branch,” reminded Judge Thomas B. Griffith, a former committee counsel for the Senate, the other half of the legislative branch.

In a sign of his comfort in court, Letter offered levity in a nod to the bodies mired in constitutional conflict.

“Judge Griffith,” Letter said, “I was hoping you’d take up the mantle of the Senate and we could complete this.”

Letter’s adversaries in court are often former colleagues, allowing him to anticipate the views of the executive branch and to speak with institutional background.

“Presidents from George Washington on (unlike Mr. Trump) have understood that Congress has the power to conduct investigations and oversight of the president as part of the constitutional design,” Letter wrote in a court filing asking the appeals court in Washington to uphold a House subpoena for Trump’s accounting firm records.

The Justice Department’s arguments are “fabricated out of whole cloth: they may represent what the department wishes the law were,” he wrote, “but they are not the law.”

Trump’s lawyers accuse the House and its lawyers of seeking the president’s financial information to embarrass the president and score political points — and of overstepping into a law enforcement role.

“Given the obvious temptation to investigate the personal affairs of political rivals, subpoenas concerning the private lives of presidents will become routine in times of divided government,” warned Trump’s attorney William S. Consovoy in asking the Supreme Court to review a ruling against the president.

Even in the middle of a bitter fight, Letter’s legal positions are distinct from his personal relationships. Halfway through the November hearing over grand jury materials, it was Letter’s turn to speak.

Letter would eventually tell the court that morning why the House had an urgent need to see the evidence: “Did the president lie?” Letter said. “Was the president not truthful in his responses to the Mueller investigation?”

But first, he made a personal point: The Justice Department attorney on the opposing side, Mark Freeman, was Letter’s former colleague and had succeeded him in leading the appellate office in part because of Letter’s “very strong recommendation.”

“After this is all over,” Letter told the judges, “we will shake hands and hug.”

Ellen Nakashima contributed to this report.

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