WASHINGTON — House Democrats are suing to force the Treasury Department to turn over President Donald Trump's tax returns.

The lawsuit, filed Tuesday in federal district court in Washington, DC, challenges Treasury Secretary Steven Mnuchin's refusal to comply with a request by the House Ways and Committee for six years of Trump's personal and business tax returns. In a letter sent to the committee in May, Mnuchin — backed by the Justice Department — found that the committee lacked "a legitimate legislative purpose."

Democrats contend that federal law doesn't require them to give a reason for asking for presidential tax returns, but that the need for Trump's returns was "evident" given congressional investigations into Trump's compliance with US tax laws and the IRS's handling of the president's returns.

"In refusing to comply with the statute, Defendants have mounted an extraordinary attack on the authority of Congress to obtain information needed to conduct oversight of Treasury, the IRS, and the tax laws on behalf of the American people who participate in the Nation’s voluntary tax system," Democrats argued in the complaint.

The case marks the latest effort by House Democrats to go to court to try to compel the release of records related to Trump and his administration. Democrats are already suing to force the release of records from Trump's accounting firm as well as financial institutions that have done business with Trump, his company, and his family.

Democrats and Trump's opponents have long clamored for the release of his tax returns. In a break with recent presidential campaign practices, Trump refused to release his tax returns leading up to the 2016 election, citing a pending IRS audit. He's also refused to make them public since taking office.

Ways and Means Committee Chair Richard Neal submitted a request to the Treasury Department for the president's tax returns in early April. The committee asked for records from 2013 through 2018, writing in a letter to the agency that it was "considering legislative proposals and conducting oversight related to our Federal tax laws, including, but not limited to, the extent to which the IRS audits and enforces the Federal tax laws against a President."

In addition to the returns, the committee also asked for other "administrative files" related to the returns and a statement about whether any of the returns was ever subject to an audit.

Mnuchin responded a month later, calling the request "unprecedented" and writing that it posed "serious constitutional questions, the resolution of which may have lasting consequences for all taxpayers." He denied the request, citing guidance from the Justice Department, writing that the US Supreme Court had held that congressional demands for information had to serve a "legitimate legislative purpose" — and the Ways and Means Committee request did not meet that standard, Mnuchin concluded. The committee formally issued subpoenas for the records soon after, which the agency did not comply with.

Last month, the Justice Department released a 33-page legal opinion from the Office of Legal Counsel explaining its reason for advising Mnuchin to deny the committee's request. The head of the office, Steven Engel, wrote that there were "marked discrepancies in the public record" about Neal's motivation for asking for Trump's tax returns, highlighting statements Neal made before Democrats regained control of the House in the 2018 midterms calling for the public release of the documents.

Engel acknowledged that federal law doesn't require congressional committees to give a reason for a request for information from the executive branch, but wrote that any request did have to serve a "legitimate legislative purpose."

"Under the circumstances, we agreed that it was reasonable to conclude

that the Committee’s asserted interest in the IRS’s audit of presidential

returns was pretextual, and that the true aim was to make the President’s

tax returns public," Engel wrote.

Democrats so far have had success in court enforcing subpoenas for Trump's financial records, although those cases involved subpoenas of private financial institutions and the president's accountants, not a request to a federal agency. Two federal judges — one in Washington and one in Manhattan — have rejected arguments by the Justice Department that Democrats' demands lacked a "legitimate legislative purpose."

In the DC case, US District Judge Amit Mehta wrote in a May opinion that the House Oversight Committee offered "facially valid legislative purposes" in subpoenaing Trump's accounting firm, and that it wasn't the court's role "to question whether the Committee's actions are truly motivated by political considerations."