“The legitimate legislative purpose is to investigate criminal behavior by the Executive branch.”

That’s not in the Ways and Means Committee’s jurisdiction, but the law they are citing empowers only the chair of that committee to demand a tax return. The legal argument against the demand is that the officer and the officeholder are logically distinct legal entities. The officer can demand a tax return to do his official duties (which, in the case of the chair of the W&M Committee, do not include investigating misconduct by the President), but the officeholder cannot demand a tax return for political purposes just because he holds the office. If the Committee were clever, they would do a compliance review of all returns of a wide class of taxpayers with lots of real estate deductions. Who knows what they might turn up?

Meanwhile, the other committees have the right to demand DJT’s return as they do any other taxpayer’s, but that authority does not extend to ordering the IRS to turn it over. If there were a legitimate basis to demand the returns, Congress would be able to order the taxpayer to produce them. The taxpayer’s failure to comply would be no different from any other act of contempt, which is to say, not something Congress can do anything about in the case of a member of the Executive branch.

Congress has no power to “investigate criminal behavior by the Executive branch.” That power lies with the Executive branch. Congress has the power to impeach, but the best way to analyze inter-branch warfare is to assume in turn that each side is malicious and the other virtuous. What rule would protect us from a bad President and also from a bad Congress? Not so easy a question. Indeed, one of the worst thing about having so unprecedentedly bad a President is the temptation to create awful precedents to deal with him.