Progressive groups renewed their calls Thursday for the leading Democrat on the House committee with jurisdiction over taxes to “immediately” obtain Donald Trump’s tax returns, which the president refused to release during the 2016 campaign after initially promising to do so.

“As the newly appointed Chairman of the House Committee on Ways and Means, it is your constitutional duty and responsibility to conduct effective oversight of President Donald Trump and his administration,” three leading progressive groups on the issue wrote in a letter, first seen and reviewed by Roll Call, to Democratic Rep. Richard Neal of Massachusetts.

The progressive coalition is composed of the Tax March, which organized rallies in 200 cities in April 2017 to demand that the president release his taxes, and anti-Trump groups Indivisible and Stand Up America.

Trump’s political opponents, including many Republicans, have for years demanded that he release his tax returns so that the American public can determine what kinds of conflicts of interest his business empire presents.

Democrats want to see what financial ties, if any, Trump has to Russia and probe whether the president fraudulently avoided paying some taxes.

Since at least 2016, Trump has said he will not release his taxes because he is under audit, though he admitted in November that does not legally prevent him from disclosing them publicly.

Neal has indicated that he will not immediately pounce on the president’s tax returns, even though his committee has the authority to obtain any U.S. citizen’s taxes through the Treasury Secretary.

Neal wants to “lay out a case about why presidents should be disclosing their tax returns before he formally forces him to do it,” one of his aides told Politico earlier this month.

The aide also hinted that aggressively pursuing Trump’s taxes risks triggering the president’s penchant for vengeance early on and predisposing him not to work with Democrats on tax policy.

“He is a very policy-driven person,” the aide said of Neal, “and I think he sees that if we break the glass and pull that alarm, you won’t get anything done after that.”

Progressive leaders have said that plan to prolong the process is unacceptable.

“Democrats pledged to obtain Donald Trump’s tax returns as part of their oversight agenda, and we will hold them accountable for that promise,” said Maura Quint, executive director of Tax March. “There is no need to slow-walk this. We have played this game for years, and Donald Trump will not willingly release his tax returns. The only solution is for Chairman Neal to request his tax returns immediately.”

Along with their letter to the chairman, the progressive groups on Thursday included a draft letter that Neal could send to Treasury Secretary Steve Mnuchin demanding that he hand over Trump’s tax returns.

That letter cites a clause in the Internal Revenue Code that requires Mnuchin to “furnish [the committee] with any return or return information” that the committee demands.

A spokesperson for the Treasury Department told HuffPost in 2018 that it would review any request for the president’s returns “for legality,” suggesting that the administration could fight the committee’s demand and the letter of the law in court.

The request for Trump’s taxes will trigger a “long and grinding legal case,” Neal told the Worcester Telegram and Gazette in an interview posted to YouTube on Wednesday.

“We are now in the midst of putting together the case,” Neal said.

Trump’s refusal to release his taxes to the public flouts presidential custom: All other major party nominees for president since Gerald Ford have willingly released their tax returns, including Democratic nominee Hillary Clinton in 2016.

The tradition of releasing tax returns began in 1973 when President Richard Nixon released his while under audit, leading to one of 37th president’s most famous quotes.

“People have got to know whether or not their president is a crook,” Nixon told reporters then. “Well, I am not a crook.”

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