The tax returns may be a different matter since they get filed with the feds anyway. But the rest of the docs they want would seem to be private and that implicateds the Fourth Amendment.
The IRS has full power to do audits. 26 USC 6103 gives Animal House access to ANY taxpayer’s returns. Is it a good thing for Nadler to have our tax returns too? Maybe congress should amend that law, since its only uses I can see are political harrassment (against conservatives), propaganda, fundraising, and pushing impeachment for no good reason.