The IRS has no choice but to turn over the tax returns.
U.S. Code Title 26. INTERNAL REVENUE CODE
(f) Disclosure to Committees of Congress
(1) Committee on Ways and Means, Committee on Finance, and Joint Committee on Taxation
Upon written request from the chairman of the Committee on Ways and Means of the House of Representatives, the chairman of the Committee on Finance of the Senate, or the chairman of the Joint Committee on Taxation, the Secretary shall furnish such committee with any return or return information specified in such request, except that any return or return information which can be associated with, or otherwise identify, directly or indirectly, a particular taxpayer shall be furnished to such committee only when sitting in closed executive session unless such taxpayer otherwise consents in writing to such disclosure.
(2) Chief of Staff of Joint Committee on Taxation
Upon written request by the Chief of Staff of the Joint Committee on Taxation, the Secretary shall furnish him with any return or return information specified in such request. Such Chief of Staff may submit such return or return information to any committee described in paragraph (1), except that any return or return information which can be associated with, or otherwise identify, directly or indirectly, a particular taxpayer shall be furnished to such committee only when sitting in closed executive session unless such taxpayer otherwise consents in writing to such disclosure.
(3) Other committees
Pursuant to an action by, and upon written request by the chairman of, a committee of the Senate or the House of Representatives (other than a committee specified in paragraph (1)) specially authorized to inspect any return or return information by a resolution of the Senate or the House of Representatives or, in the case of a joint committee (other than the joint committee specified in paragraph (1)) by concurrent resolution, the Secretary shall furnish such committee, or a duly authorized and designated subcommittee thereof, sitting in closed executive session, with any return or return information which such resolution authorizes the committee or subcommittee to inspect. Any resolution described in this paragraph shall specify the purpose for which the return or return information is to be furnished and that such information cannot reasonably be obtained from any other source.
It says "shall" not if, and or but. They will have to comply or go to jail.