A team from Jenner & Block has formally joined the U.S. House's lawsuit seeking to compel testimony from former White House counsel Don McGahn, as the case heads to en banc arguments before the U.S. Court of Appeals for the D.C. Circuit later this month.

Matthew Hellman, co-chair of the firm's appellate and Supreme Court practice, and Elizabeth Deutsch, formerly a fellow with the ACLU, filed a notice of appearance in the case Thursday. Their names were also both included on a brief filed by House lawyers later that afternoon.

The House has regularly received pro bono assistance from Big Law in its litigation against the Trump administration. A team from Sidley Austin, for example, is working on the House's suit challenging the administration's diversion of military funds for border wall construction, which is also set to be heard by the en banc D.C. Circuit later this month.

The House is now seeking a ruling from the en banc D.C. Circuit that lawmakers do have standing to go to court to enforce subpoenas, after a divided panel found earlier this year they did not. The D.C. Circuit vacated the panel opinion in granting the en banc argument.

The case also hinges over whether McGahn has "absolute immunity" from testifying before Congress, as President Donald Trump claims the former White House lawyer does. But focus has shifted toward the standing issue, in light of the panel opinion, which could have broad ramifications for the House's other active lawsuits against executive branch officials.

In the brief filed in the McGahn lawsuit Thursday, lawyers for the House—which also include attorneys from the House's Office of General Counsel and Georgetown Law's Institute Constitutional Advocacy and Protection—took issue with several parts of the panel opinion in McGahn.

They wrote that few lawsuits have been previously filed by Congress against the executive branch because "early Congresses generally had no need to sue because the executive traditionally cooperated with congressional inquiries—either in full or after a compromise that gave Congress much or all of the necessary information."

"President Trump, by contrast, has broken with his early predecessors and declared McGahn absolutely immune from testifying. And although McGahn maintains that an accommodation was reached regarding the documents the committee seeks, the executive has disclosed nothing to date," the brief states.

The lawyers also refuted the panel opinion's assertion that the subpoena fight is "deeply political" and therefore doesn't belong in court, arguing that judges "must ensure that each branch may exercise the powers the Constitution accords it, so that political battles occur on the playing field the Constitution establishes."

The filing also referenced the recent impeachment of Trump, which was brought forward by House Democrats. House general counsel Douglas Letter, as well as other House attorneys, were part of that effort. The Senate in February acquitted Trump of the charges of obstruction of Congress and abuse of power.

House lawyers on Thursday said, even after impeaching Trump, "the House still has not received the information it seeks" over a hold on Ukrainian military aid and Trump's push for investigations into the Biden family.

"Indeed, President Trump's lawyers defended him against obstruction-of-Congress charges in the Senate by arguing that the House should have first sought judicial enforcement of its subpoenas—even as the executive told this court that judicial enforcement of subpoenas would be unconstitutional," they continued.

The lawyers also wrote the House "should have access to full information before taking the drastic step of impeaching the president" in arguing lawmakers should not use the tool frequently. Republicans criticized Democrats for not obtaining more documents before drafting and passing articles of impeachment against Trump; Democrats replied by saying Trump was a threat to the upcoming 2020 election and they could not wait for time-consuming litigation to play out over the requested documents.

"McGahn's argument reflects a pattern of nonaccountability. Time and again, the executive resists congressional oversight by insisting that Congress may hold it accountable using other tools. But when Congress invokes those tools—the contempt power, the appropriations power, or even the impeachment power—the executive insists that those tools are likewise improper," House lawyers wrote Thursday.

"The system McGahn envisions is one that leaves Congress effectively powerless to hold the executive accountable and would be unrecognizable to the Framers who designed our system of checks and balances," they concluded.

A handful of amicus briefs were also filed in the case Thursday in support of the House.

Attorneys from Arnold & Porter Kaye Scholer filed a brief on behalf of a bipartisan group of former members of Congress, and Andrew Herman of Miller & Chevalier filed another for the Lugar Center and the Levin Center at Wayne Law.

Lawyers at Lieff Cabraser Heimann & Bernstein also filed a notice of appearance in the case, indicating they would be filing a brief for Harvard's Laurence Tribe and former Democratic Rep. Elizabeth Holtzman, who, as a member of the House Judiciary Committee, voted in favor of articles of impeachment against President Richard Nixon. Orrick, Herrington & Sutcliffe filed an amicus brief for legal scholars.

A group of former Justice Department officials, represented by the Constitutional Accountability Center, also filed a brief Thursday. The ex-DOJ officials took issue with the Trump Justice Department's current position against the House's ability to go to court over subpoena enforcement.

"Historically, the executive branch took the position that Congress can seek civil judicial enforcement of its subpoenas against executive branch officials. Indeed, it relied on the existence of that authority to justify its position that Congress cannot invoke criminal contempt procedures or its inherent contempt authority against a recalcitrant executive branch official when the president asserts executive privilege," the former officials said. "The department's more recent position is at odds with this precedent."