WASHINGTON — Soon after the Democrats take control of the House next year, Rep. Richard Neal, who will be the chairman of the House Ways and Means Committee, expects to send a letter to Treasury Secretary Steve Mnuchin requesting copies of Donald Trump's returns.

He doesn't expect to get them.

At least, not right away.

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A 1924 federal law — 26 U.S. Code § 6103 — mandates that the Treasury secretary "shall furnish" the tax returns of any individual for private review by the chairmen of the House and Senate tax committees. Committee sources could find no evidence that it had ever been used to obtain somebody's tax return.

But they say the law, which was passed to monitor conflicts of interest in the executive branch, is clear.

Neal, of Springfield, Massachusetts, has said he intends to use the law to seek Trump's tax returns, because he believes it's wrong that Trump is the first president in 40 years not to have made his returns public.

Rep. Richard Neal, incoming chairman of the House Ways and Means Committee, with staff member Margaret Boyle before a meeting in Boston on Nov. 27, 2018. Steven Senne / AP file

But Neal has said he also expects that the Treasury Department — and perhaps Trump himself — will put up a ferocious legal fight.

"I assume that there would be a court case that would go on for a period of time," Neal told reporters recently.

If and when he does get them, Neal and the Democrats plan to make the returns public, Congressional sources told NBC News. They believe they can do so by committee vote in a closed session.

If that happens, some will see a fearsome abuse of government power, while others will welcome an important check on a president who has refused to play by accepted ethical rules.

If Neal does get the returns, he won't be the first outside Trump's orbit to see them, legal experts say.

They say it's a near certainty that special counsel Robert Mueller's team, and perhaps federal prosecutors in Manhattan, long ago obtained Trump's tax returns, meaning not just his personal returns, but those associated with every company he controls.

"I think it's virtually certain that Bob Mueller has obtained them," said Dan Goldman, a former prosecutor and NBC News analyst.

"It's Prosecution 101 — you get the credit reports and you get the tax returns," said Chuck Rosenberg, a former federal prosecutor and NBC News legal analyst. "Because both of those documents give you leads to other documents. Those are incredibly rich documents and also incredibly easy to get."

Prosecutors can get tax returns through a judge's order, as long as they convince the judge that the returns are relevant to their investigations. They can go back 10 or even 20 years, if the investigation demands it.

But unlike Congress, prosecutors who get tax returns are bound by strict secrecy rules. They can't make them public unless they become evidence in a trial.

One question Trump's tax returns might answer is whether he has received significant income from, or has investments with, Russians. But if such income or investments were hidden through a web of shell companies, it could take a team of forensic accountants to uncover it.

Mueller also has access to suspicious activity reports and other financial records that would allow him to track the movement of money among Trump's accounts, said Brett Kappel, a Washington lawyer who specializes in campaign finance and government ethics.

"The tax documents will stack up thicker than a phone book, but you can track how the money moved," Kappel said.