(Reuters) - President Donald Trump, his three oldest children and the Trump Organization have sued Deutsche Bank AG and Capital One Financial Corp to try to block them from responding to U.S. congressional subpoenas issued by Democrats seeking financial records.

FILE PHOTO: The Deutsche Bank headquarters are pictured in Frankfurt, Germany, April 25, 2019. REUTERS/Ralph Orlowski/File Photo

The lawsuit filed late Monday opens a new front in Trump’s battle to stop the Democratic-led House of Representatives from probing the Republican president, his family, his businesses and his administration after Special Counsel Robert Mueller finished his inquiry into Russia’s role in the 2016 election.

The lawsuit said urgent action was necessary because both banks signaled they would begin responding to the subpoenas unless a court intervened by May 6.

Deutsche Bank has long been one of the main banks for Trump’s real estate empire. Democratic lawmakers, meanwhile, asked Capital One’s chief executive in March for documents related to potential conflicts of interest tied to Trump’s hotel in downtown Washington and other business interests.

In the complaint filed in Manhattan federal court, Trump, his adult children Donald Jr., Eric and Ivanka, and the Trump Organization accused House leaders of pursuing records for no legitimate or lawful purpose, hoping they would “stumble upon something” they could use as a political weapon against Trump.

“The subpoenas were issued to harass President Donald J. Trump, to rummage through every aspect of his personal finances, his businesses, and the private information of the President and his family,” the complaint said.

Only the banks were named as defendants.

Representative Maxine Waters, who chairs the House Financial Services Committee, and Representative Adam Schiff, who chairs the House Intelligence Committee, said on April 15 their panels had issued subpoenas to multiple financial institutions for information on Trump’s finances.

“The president will do anything and everything that he can to obstruct justice ... to shut down an investigation,” Waters told reporters on Capitol Hill.

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She also said Trump has “cast a gauntlet and he has said that he is going to fight,” referring to her investigation of Deutsche Bank. “We will fight him,” Waters added.

Deutsche Bank said it was “committed to providing appropriate information to all authorized investigations and will abide by a court order regarding such investigations.”

Capital One was not immediately available for comment. Lawyers for the plaintiffs were also not immediately available.

The case was assigned to U.S. District Judge Edgardo Ramos, an appointee of Democratic former President Barack Obama.

Michael Stern, who served as senior House counsel from 1996 to 2004, said the lawsuit was unlikely to succeed because judges do not like second-guessing lawmakers’ motivations in investigations.

Stern also said Trump’s decision to sue only Deutsche Bank and Capital One appears to have been strategic because the banks would not try to pick sides and were less likely to contest an injunction blocking a subpoena.

Andy Wright, a former White House lawyer under Obama now at the law firm K&L Gates, said: “Courts have been really reluctant to assess whether or not Congress has a legitimate interest or not. They have been very deferential in the past.”

DEFYING LAWMAKERS

Trump, who is seeking re-election next year, has aggressively sought to defy congressional oversight of his administration since Democrats took control of the House in January, including possible dealings with Russia, and said “we’re fighting all the subpoenas” issued by the House.

That battle could herald a major showdown between the executive and legislative branches testing the separation of powers spelled out in the U.S. Constitution.

The White House is also resisting other House subpoenas, including for Trump’s personal and business tax returns, and sought to block current and former administration officials from cooperating with House investigators.

Waters and Schiff in a joint statement called the lawsuit part of Trump’s “unprecedented stonewalling” of congressional oversight.

In the complaint, the plaintiffs said House leaders ignored constitutional limits on Congress’ power to investigate, and any probe needed to further some “legitimate legislative purpose” and could not be an end in itself.

They also said the subpoenas violated the plaintiffs’ privacy rights, and the refusal of House committees to even provide copies of the subpoenas to the plaintiffs made it impossible to know, or negotiate, their scope or breadth.

Deutsche Bank stood almost alone in extending credit and managing money for Trump before he became president, as other banks avoided him because of his poor track record in paying back loans. The German lender has extended a substantial amount of capital to the Trump Organization in the past decade, according to documents and media reports.

A 2017 financial disclosure form showed that Trump had at least $130 million of liabilities to Deutsche Bank.

In March, the New York Times detailed other financial relationships, including more than $2 billion of property loans and private banking services.