WASHINGTON — Democrats took control of the House this year promising to use legislation and investigations to check President Trump. But facing substantial roadblocks to each, they are increasingly opposing him in a different way: Eight months into their majority, the House is going to court at a tempo never seen before.
Fighting in courtrooms as much as in hearing rooms, the House has already become a party to nine separate lawsuits this year, while also filing briefs for judges in four others. More lawsuits are being drafted, according to a senior aide to Speaker Nancy Pelosi.
The fights include efforts to reveal Mr. Trump’s hidden financial dealings, force his aides to testify about his attempts to obstruct the Russia investigation, challenge his invocation of emergency powers to spend more taxpayer money on a border wall than Congress approved and defend laws like the Affordable Care Act that his Justice Department abandoned.
While it is routine for the executive branch to be in court, it was once vanishingly rare for Congress, which has typically used its authority to pass laws, appropriate funds and investigate the executive branch, to balance out the president’s power.
Now, as Senate Republicans refuse to take up the bills they pass, Trump administration witnesses refuse to show up for their hearings and the president levels his own highly unusual lawsuits against the House’s oversight requests, two of the three branches of government are regularly facing off before the third, creating a new stress with uncertain consequences for the political system.
“It is unprecedented,” said Charles Tiefer, a former longtime House lawyer who is now a University of Baltimore law professor. “The challenges for the House counsel ebb and flow over time, but this is like nothing else in history.”
The consequences of the specific disputes could be significant. In the short term, they could determine whether House Democrats are able to drag information to light about Mr. Trump that could lead to his impeachment or damage his re-election prospects. And potential decisions by the higher courts could clarify the long-ambiguous line between a president’s secrecy power and Congress’s oversight authority — determining whether future presidents can systematically stonewall congressional subpoenas.
But the broader phenomenon is also significant.
As an immediate matter, the surge in litigation is a consequence of Mr. Trump’s norm-busting presidency. House Democrats are looking for additional venues through which to take him on — or, in some cases, fighting lawsuits that the president filed against Congress himself to try to block lawmakers from obtaining information about him from entities outside the federal government. But it is also bringing into clearer view how, over the past generation, Congress was already starting to go to court more often than had been the historical norm, as political compromise gave way to deadlock amid growing partisan polarization.
That trend line suggests that even if the number of congressional lawsuits declines when the next president takes office, the constitutional order could change in a way that Mr. Tiefer and other legal scholars view as dangerous. He said it is better if the two parties were able to resolve high-level policy disputes through compromise rather than through the “rigid and formalized system” of litigation, and suggested that routinely pushing those disputes into court could heighten politicization of the judiciary.
To handle the mounting workload, the House’s general counsel, Douglas Letter, a former Justice Department litigator, and his staff of seven lawyers have increasingly relied on volunteer lawyers at white-shoe law firms and at public interest groups — including several prominent veterans of the Obama legal team — to help research and draft court filings.
Representative Adam B. Schiff, Democrat of California and the chairman of the Intelligence Committee, which was a party in one of the subpoena lawsuits for Mr. Trump’s banking records, portrayed the increasing litigation as an unfortunate necessity.
“The blanket refusal to comply with any legitimate process has forced us to go to court to validate Congress’s power of oversight,” he said. “If we don’t, we are at risk of losing that power, and that would be a tragedy for the country because it would take any limit off the executive.”
Republicans say it is the Democrats who are out of control and violating traditional norms of governance. The Justice Department has accused the House of asking the judiciary “to take its side in political disputes” and of trying to “use federal courts to accomplish through litigation what it cannot achieve using the tools the Constitution gives to Congress.”
For most of American history, Mr. Tiefer said, Congress never went to court over disputes with the executive branch. The seeds of change began during the Watergate scandal, when Congress enacted a special law enabling a Senate committee to sue President Richard M. Nixon to try to gain access to his Oval Office tapes.
In the post-Watergate reform era, the House created a small general counsel office, raising the possibility of filing civil lawsuits seeking enforcement of its subpoenas to executive branch officials if a president tried to block them. But that threat remained essentially theoretical for more than a generation, as the two branches resolved such disputes through negotiations.
But in 2008, House Democrats went to court to compel disclosure of information from the Bush administration about its firing of a group of United States attorneys. In 2012, House Republicans sued for internal Obama Justice Department documents related to the botched gun-trafficking case known as Operation Fast and Furious.
Those rare subpoena-related cases have now grown routine. Already this year, Democrats have filed lawsuits seeking to enforce subpoenas for Mr. Trump’s federal tax returns and for testimony from Donald F. McGahn II, his former White House counsel and a key witness to several obstruction episodes in the special counsel’s Russia investigation report.
Separately, Mr. Trump’s personal lawyers have filed lawsuits against the House seeking to block financial firms —Mazars USA and Deutsche Bank and Capital One — from complying with its subpoenas for his business records. Mr. Trump has also filed one against the House to block it from requesting his state tax returns from New York.
Several more subpoena-related lawsuits are under development, including most likely a suit for executive branch information that could reveal the administration’s motivation for trying to add a citizenship question to the census.
The House has also filed litigation asking a judge to grant the House Judiciary Committee access to secret evidence Robert S. Mueller III, the former special counsel, gathered using a grand jury. That effort draws on a Watergate-era precedent, but the House was not itself a party to the 1974 request.
Also on an upward trend are cases in which the House is intervening in court to defend statutes because the executive branch refuses to do so, contrary to the Justice Department’s longstanding role of defending acts of Congress that come under constitutional challenge.
Although there have been several lawsuits in the past where Congress stepped in to defend a law that the executive branch said permitted lawmakers to unconstitutionally encroach on presidential power, it is now starting to become more common for the Justice Department to refuse to defend a law — and for the House to step in — without a separation-of-powers rationale.
In 2011, House Republicans intervened to defend a law that barred federal recognition of same-sex marriages that were lawful at the state level after the Obama administration deemed it unconstitutional and stopped defending it in court. At the time, such an intervention by Congress was highly unusual.
But this year, it has already happened twice. The Justice Department under Mr. Trump has refused to defend the Affordable Care Act and a law against female genital mutilation. The House has sought to intervene and provide lawyers to argue that both are constitutional.
It is also becoming more common for the House to sue the executive branch over spending and policy disputes. In 2014, the Republican-controlled House filed an unusual lawsuit over how the Obama administration was putting the Affordable Care Act into effect, including its provision of insurer subsidies the lawmakers said were unauthorized.
Echoing that move, House Democrats this year filed a lawsuit challenging Mr. Trump’s plan to use emergency powers to spend more on a border wall than Congress appropriated for that purpose. A district court judge ruled that the House lacked standing, and it has appealed.
Across this array of litigation, Mr. Letter — a former Justice Department civil litigation specialist — has handled courtroom appearances. But his office has been working with outside lawyers for research and brief drafting.
The aide to Ms. Pelosi said that all of the outside legal services have been provided for free, in contrast to the several million dollars that House Republicans spent on outside lawyers for the marriage law case.
The lawyers volunteering to help the House Democrats include Donald B. Verrilli Jr., a former solicitor general, and a team at his firm, Munger, Tolles & Olson; Virginia A. Seitz, a former head of the Justice Department’s Office of Legal Counsel, and others at her firm, Sidley Austin; Neal K. Katyal, a former acting solicitor general and a partner at Hogan Lovells; and Georgetown Law Center’s Institute for Constitutional Advocacy and Protection, run by two former national security officials, Joshua A. Geltzer and Mary McCord.
Beyond immediate political consequences, legal specialists said that the fate of the litigation wave might carry broader implications for Congress’s ability to counterbalance the presidency in the future, regardless of which party was in power.
Kerry W. Kircher, who served as House counsel under Republican speakers between 2011 and 2016, said many of the House’s lawsuits were justified, regardless of politics, to ensure that Mr. Trump’s vow to defy “all” of its subpoenas did not set a precedent.
House Democrats “are trying to do what they think they were elected to do and make sure that the House in the future will be able to conduct oversight,” he said. “What is going on with this administration is no less than an all-out declaration of war on oversight.”