In the latest case, Trump, his family and the Trump Organization have filed a lawsuit against Deutsche Bank and Capital One attempting to thwart congressional subpoenas into his financial and business dealings, asserting the requests are out of bounds.
That comes as Trump's treasury secretary is declining to produce the president's tax returns, Attorney General William Barr is threatening to back out of his agreement to appear this week before the Democratic-led House Judiciary Committee, and former White House counsel Don McGahn and other officials are being encouraged not to testify before Congress.
"He's prepared to fight us tooth and nail. And we're prepared to fight him back," said Rep. Maxine Waters, D-Calif., the chairwoman of the Financial Services Committee. "He obviously has something to hide."
The standoff pits the legislative and executive branches against each other in a constitutional showdown not seen since the Watergate era. Neither side is expected to back down. The debate over witnesses and documents could escalate with legal battles rippling into the 2020 election.
From Trump's perspective, since Mueller finished his report on Russian interference into the election, there's no further need to investigate. It's a view largely backed by the president's party in Congress. But Democrats say it's their duty to conduct oversight even as they are also confronting the limits of their own enforcement powers.
Rep. Jerrold Nadler, D-N.Y., the chairman of the Judiciary Committee, said the stonewalling "certainly builds the case that the administration and the president is engaged in wholesale obstruction of Congress, completely extraconstitutional, trying to make the presidency not responsive to Congress, trying to make the presidency into a monarchy."
Nadler said the White House's position is "absolutely unacceptable." Impeachment proceedings, though, which would run through Nadler's committee, remain off the table for now, as Speaker Nancy Pelosi is urging the House chairmen to push forward with their oversight agendas.
Republicans have largely stood by Trump and shown little interest in the oversight agenda many view as little more than a partisan attack on the president. Senate Majority Leader Mitch McConnell, in his first remarks in Washington since the special counsel's report was released almost two weeks ago, said he "didn't hear a single word about the Mueller report" from constituents at home in Kentucky.
McConnell brushed off concerns that Trump's decision to ignore congressional subpoenas could set a precedent for executive overreach by this White House or future ones. "Every administration since I've been around has been in disputes with Congress over power," McConnell told reporters. "We'll see how it all sorts out."
Congress has a range of tools available to try to force compliance from the White House, either through civil lawsuits compelling administration officials to testify or produce documents, or by holding others in contempt of Congress, as it seeks information for investigations stemming beyond the special counsel's probe.
Mueller's nearly two-year investigation left unanswered a key question of whether the president obstructed justice. While the report did not find that Trump conspired with Russia to interfere in the 2016 election, it recounted 10 instances where Trump tried to intervene in the investigation.
Barr is set to testify Wednesday in the Senate, but his appearance Thursday in the House is uncertain. House Democrats have also asked Mueller to testify by May 23, but Republicans, who have the majority in the Senate, have not made a similar request.
Sen. Ted Cruz, R-Texas, a member of the Judiciary Committee, said he was unsure if Mueller should testify. While Congress has a "legitimate and important" oversight role, he said, "it's also hard to see the Democrats are exercising that function in good faith when their only objective is trying to destroy the president."
For Democrats, the ability to conduct oversight of the White House is a core responsibility that extends beyond investigating the president into agency actions that can touch the lives of Americans. "If the executive branch can deny the legislative branch the ability to bring witnesses to testify under oath and for the production of documents, the executive branch will have essentially eliminated the oversight function of Congress," said Rep. David Cicilline, D-R.I.
Yet while Democrats have vowed to go to court, those proceedings could last years, possibly past Trump's tenure. And if they chose to hold officials in criminal contempt, which would take a vote of the full House, it would be referred to Justice Department officials unlikely to side with the Democrats.
Some Democrats have thrown out other options: daily fines for not showing up, for example, or cutting appropriations for an official's agency. But those ideas might not be politically popular. There's also an option that would be even more contentious and hasn't been used in decades — trial and even imprisonment by Congress. Called "inherent contempt," this process was often used in the country's early years but hasn't been employed in almost a century. While Democrats have vowed to use all of the available legal tools, they have shown no interest in going that far.
Despite drawbacks, Democrats say they will have to fight on multiple fronts to get the witnesses and documents they need. "If you let them get away with this, then what do you have?" said House Oversight and Reform Committee Chairman Elijah Cummings on Monday. "If the president can get away with blocking any information and anybody from testifying before the Congress, what road are we going down?"
Associated Press reporter Padmananda Rama contributed to this report.