Democrats, eyeing control of a powerful House tax-writing committee next year, are studying a century-old provision in the federal tax code that could give them access to President Trump’s long-sought tax returns and eventually the ability to make them public.

The powers laid out in an obscure 1920s addition to the tax code are clear: The leaders of Congress’s tax-writing committees, including the Ways and Means Committee in the House, are empowered to request from the Treasury Department tax returns or related information on any tax filer. Democrats could use that information to finally determine if Mr. Trump, who built a global business empire before entering politics, has problematic financial entanglements with Russia or other undisclosed conflicts of interest….

The provision — Section 6103 in the tax code — dates to the early 1920s, when Congress was mired in investigations of bribes supposedly paid to officials in the Harding administration by citizens seeking leases of federal lands containing rich oil reserves. Lawmakers found themselves reliant on the president to release tax information on officials accused of wrongdoing and argued that to uphold the proper separation of powers and serve as a check on the executive branch, they needed authority to obtain tax returns on their own.

“The whole tenor of the debate was not that different from today’s debate,” said George K. Yin, a University of Virginia tax law professor.

In the decades since, the authority has been used scarcely. In 1974, one committee used it to supplement tax information provided by President Richard M. Nixon to investigate whether he had taken improper tax positions and later to release a nearly 1,000-page bipartisan report on its findings.

In 2014, in a more partisan push, Republicans on the Ways and Means Committee used it to obtain and publicly release tax information as part of an investigation into whether the I.R.S. discriminated against conservative entities seeking tax-exempt status. Democrats denounced it as a political smear that abused the committee’s authority.