"It's a messaging document that doubles down on Trump's notion that this is all a witch hunt," Dallek said.
The president is following the standard practice of trying to delay and stall, said University of Virginia law professor Saikrishna Prakash.
"What's different here is that Congress has subpoenaed a third party," he said.
"I think that's what's got them worried."
President Richard Nixon sought to quash congressional subpoenas seeking tape recordings of Oval Office phone conversations, both Dallek and Prakash noted. But those were official government records and not privately held by a third party, the UVA professor said.
Cummings laid out his case for the committee's authority to pursue the financial records from Mazars in an April 12 memo to House Oversight members.
"The committee has full authority to investigate whether the president may have engaged in illegal conduct before and during his tenure in office," Cummings wrote. He said it's also the committee's job to determine whether Trump has undisclosed conflicts of interest, or is accepting gifts from foreign or other governments in violation of the Constitution.
"The committee's interest in these matters informs its review of multiple laws and legislative proposals under our jurisdiction, and to suggest otherwise is both inaccurate and contrary to the core mission of the committee to serve as an independent check on the Executive Branch," Cummings wrote.
Trump's lawsuit comes less than a week after the public release of special counsel Robert Mueller's partly redacted report on his two-year probe of Russian interference in the 2016 presidential election and possible coordination with the Trump campaign.
Though the 448-page report detailed extensive contacts between Russians and people connected to Trump's campaign, Mueller said facts uncovered during the investigation -- which also addressed the president's pursuit of a Trump-branded tower in Moscow while campaigning -- didn't establish a criminal conspiracy.
"The fact is Trump was seeking to do business in Russia," Dallek said, adding that the president's been implicated in potential campaign finance violation, his New York-based charity closed down due to allegations of illegal activity and he's been accused of profiting from foreign governments through his Washington hotel in violation of the constitution.
"There's probably a half dozen instances where his private business concerns are connected directly or indirectly to his public responsibilities as president, Dallek said. "There are legitimate questions."
(Joe Light and Laura Davison contributed to this report.)
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