For all the attention given to the felonies committed by Michael Cohen and Paul Manafort, the Mueller investigation still hasn’t shined a light on the central mystery surrounding Donald Trump's presidency thus far: Just what, if anything, he owes to Russia.
<snip>

...according to the rules already on the books, the Senate doesn’t need a new law to see Trump’s returns. Rather, action by a single Senate Republican may be all that is needed to initiate an immediate investigation of Trump’s tax returns and begin the process of discovery.

According to a law passed in 1924, in part to address concerns about conflicts of interest involving executive branch officials such as treasury secretary Andrew Mellon, the House Ways and Means Committee, the Senate Finance Committee, and the Joint Committee on Taxation have the right to obtain from the IRS any person’s tax returns—including the president’s—without his consent and to conduct an investigation. If there is a legitimate purpose, the committees may then release the tax information to the Congress for potential disclosure to the public.

Right now, the Republicans hold a mere one-vote majority in the Finance Committee, which means consent by a single Republican on that committee could be enough to authorize committee action. - Politico