Law prof George Yin has written a brief and timely paper called Congressional Authority to Obtain and Release Tax Returns. Yin is the former chief of staff of Congress's Joint Committee on Taxation.
In his paper, Yin explains that particular congressional committees possess the power to obtain, review, and disclose filers' tax returns in limited circumstances and that, in Trump's case, the committees should exercise at least their obtain-and-review authority:
In the present situation, concerns over President Trump’s possible conflicts of interest — including conflicts with tax legislation the committees may soon be asked to approve — would certainly seem to justify a tax committee effort to obtain and inspect his confidential tax information. A review could also assure the American public that the IRS is treating him like any other taxpayer and not giving him preferential treatment. … Whether there should be disclosure of any information obtained is a separate question. There must also be a legitimate purpose for any disclosure. Once the tax committees complete their investigation, they can determine whether any disclosure to the public would be appropriate.
Here is the abstract of Yin's paper:
President Trump's continuing refusal to release his tax returns despite the contrary common practice of presidents over the last 40 years has spurred interest in finding alternative ways to obtain the information. This article describes the authority of the tax committees in Congress to obtain, inspect, and disclose the confidential tax information of any taxpayer, including the president, without the taxpayer's consent.