Washington (CNN) The request by House Democrats for six years of Donald Trump's tax returns opens a serious new political, legal, constitutional and personal front in the party's battle to use its new majority to check a defiant President.

It is a bid to solve one of the most tantalizing and enduring mysteries of Trump's 2016 campaign and his presidency -- what is it in the President's unseen financial and business life that he doesn't want Americans to see?

The historic move, made official by House Ways and Means Committee Chairman Richard Neal on Wednesday, is all but certain to unleash a legal battle narrowing in on Trump's most guarded secrets that could drag on for months or even years while shaping the terrain of the 2020 election.

The confrontation, will in its broadest sense, establish the extent to which Congress has the power to examine the executive and enforce a long-observed norm -- that party nominees and Presidents release their returns to the public.

But Democrats say this is not a matter of choice, it's the law, as they yet again seek to straddle the fine line between legitimate oversight and the risk of being seen as overreaching in a way that could rebound against them politically.

The request represents a remarkable moment and another monument to the unconventional nature of the Trump era, since no Congress has attempted such a challenge before -- simply because no President in recent times has resisted such a basic test of transparency.

Neal, the only Democrat on Capitol Hill with the power to take such action, is using a little-known IRS code in requesting six years of Trump's personal returns and also wants records from eight of the President's business entities. His request is narrowly targeted and rooted in a legal technicality.

But supporters of the drive to get clarity on Trump's taxes argue that it is vital for Congress to establish whether Trump has any conflicts of interest that bear on his official duties and to see whether he is getting payments from foreign states in violation of the emoluments clause of the Constitution.

Democrats have also justified an effort to open Trump's returns as a way to examine how much he and his businesses -- from which he is not fully divested -- benefited from the massive GOP tax reform law.

Trump has also been accused, notably by his former attorney Michael Cohen, of both deflating and over estimating the value of his properties and assets in insurance claims and on loan applications — a practice on which tax returns could shed light.

'Within our ... rights'

Neal, who was facing increasing pressure from liberal activists to speed up a meticulous process of requesting the returns, insisted his move was about "policy not politics," in a letter to the IRS.

"I am certain we are within our legitimate legislative, legal and oversight rights," said Neal, a 16-term congressman who is not well known outside his home state of Massachusetts or the halls of power on Capitol Hill, said in a statement.

Neal, 70, will now shed his relative obscurity and become one of the top political generals in the Democratic battle against Trump -- and will have to accept all the vitriol, attacks and mockery that comes with taking on the President.

Though he insisted his move was not political, it will inevitably be seen in the context of a widening use of Democratic muscle following the party's capture of the House in November.

Just this week, House committee chairs have readied subpoenas to demand the full release of Robert Mueller's Russia report and to examine accusations of abuses in the White House's handling of top secret security clearances.

Neal's intervention came less than two weeks after Mueller ended his investigation and a summary by Attorney General William Barr said the special counsel did not establish there was cooperation between Trump's team and Russia's election meddling scheme or decide whether the President obstructed justice.

The request dampened the hopes of liberals in the party's grass roots that an impeachment saga could be looming. The New York Times, however, reported on Wednesday evening that some of Mueller's investigators have told associates that the findings of the original report are more damaging for the President than Barr indicated.

Trump shrugged off the Democratic request for his taxes on Wednesday evening, using similar phraseology as when he was asked during the campaign about his taxes.

Asked whether he would comply with the Democratic request, Trump said that "until such time as I'm not under audit I would not be inclined to do that."

Another unreasonable scrutiny?

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Republicans are certain to argue that the request for Trump's taxes is yet another example of unreasonable scrutiny by Democrats frustrated with indications that Mueller's report will not doom Trump's presidency. One reason that the request appears to be comparatively narrow -- for returns between 2013 and 2018 -- could be to confine the search to the period immediately before and during Trump's political career and to stave off accusations that the Democrats are embarking on an illegitimate fishing expedition.

The drama threatens to draw in some key members of the Trump administration -- including Treasury Secretary Steven Mnuchin.

Mnuchin said at a congressional hearing in March that he would "comply with the law" if asked to release Trump's returns -- though also suggested the final call could rest with the commissioner of the Internal Revenue Service.

It is significant therefore that Neal's request was made to the IRS and not the Treasury.

It is certain that the President would see any decision by Mnuchin to get involved and comply with Neal's request as a flagrant personal betrayal.

Neal's gambit now puts Charles Rettig, the current IRS commissioner, who was nominated by Trump and confirmed by the Senate, in a delicate political position.

It is all but certain that the administration will take steps to challenge the Democratic interpretation of the law on which the request is based. Such a scenario could see the Ways and Means committee issue a subpoena after Neal's April 10 deadline and the unleashing of a complicated legal duel.

The fight could become a cause celebre that works its way through the court system and even reaches the Supreme Court, and the conservative majority that is newly restocked by Trump.

The committee spent weeks plotting its move in the hope of preparing the most complete case possible for the court battle to come.

Neal argued that he needed to see Trump's returns in order to properly consider legislation related to the IRS's practice of auditing sitting presidents.

"Under the Internal Revenue Manual, individual income tax returns of a President are subject to mandatory examination, but this practice is IRS policy and not codified in the Federal tax laws," Neal wrote in a letter to the IRS.

"It is necessary for the committee to determine the scope of any such examination and whether it includes a review of underlying business activities required to be reported on the individual income tax return."

Trump has long seen scrutiny of his complicated and difficult to penetrate personal finances as a red line that should not be crossed.

In addition to Trump's personal records, Neal requested information from the President's business entities, including the Bedminster golf course LLC, as well as DJT Holdings LLC, DJT Holdings Managing Member LLC, DTTM Operations LLC, DTTM Operations Managing Member Cor, LFB Acquisition Member Corp and LFB Acquisition LLC.

Neal's request is specific and under the section of the IRS code that he is using only he, and not other chairmen of House committees can see the returns.

It would require a vote in the committee to release any such documents to the public if they are eventually handed over.