Democrats in Congress long have demanded that President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE make his tax returns public. Many promised voters that, if given the House majority in the 2018 elections, they would force public disclosure of Trump’s returns. Indeed, they’ve demanded access to the president’s returns, but Treasury Secretary Steven Mnuchin Steven Terner MnuchinHillicon Valley: DOJ proposes tech liability shield reform to Congress | Treasury sanctions individuals, groups tied to Russian malign influence activities | House Republican introduces bill to set standards for self-driving cars Treasury: Trump's payroll tax deferral won't hurt Social Security Treasury sanctions individuals, groups tied to Russian malign influence activities MORE has refused to give Congress that access. He was right to refuse. His action is firmly grounded in federal statute and the Constitution.

In April, House Ways and Means Committee Chairman Richard Neal (D-Mass.) demanded Trump’s tax returns from 2013 to 2018, invoking a federal statute (26 U.S.C. § 6103) that makes federal tax returns confidential. Other statutory sections, including 26 U.S.C. § 7213, make it a felony to disclose information in federal tax returns without proper authorization.

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There are narrowly drawn exceptions to the general rule of confidentiality, including one that allows congressional tax committees to demand copies of individual tax returns. That information, however, cannot be made public without the taxpayer’s written consent. Secretary Mnuchin must have a well-grounded fear that one or more members of Congress would make the president’s returns public, hiding behind the Constitution’s speech or debate clause to escape prosecution. This factor alone can preclude the release of tax information.

There are, however, even more fundamental problems with the request. The committee’s stated purpose is to investigate how the IRS enforces tax laws against sitting presidents. That is an obvious pretext. Even if the Democrats’ posturing could be ignored, the fact that only Trump’s returns are sought — and not those of former presidents — makes the game clear.

Former presidents have disclosed some tax information, but their full returns and all supporting documents were not released. And since the ostensible oversight focus is how the IRS audits tax returns of sitting presidents, that type of information is not publicly available. In addition, even if Secretary Mnuchin were to ignore the politics involved, he would be justified in withholding the president’s tax returns on constitutional grounds.

Congressional demands for information must be grounded in proper constitutional powers. Congress does not have general investigative authority, let alone a mandate to enforce federal law, both of which are vested in the president. Nor does it have adjudicative power, which is reserved to the judiciary. Its proper investigative power is broad but limited to the purposes of legislation or oversight. And Congress’s oversight powers can be exerted only over matters that plausibly can be reached through the exercise of congressional legislative powers.

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As the Supreme Court stated in Watkins v. United States (1957), with respect to a McCarthy-era demand by the House Un-American Activities Committee for information from a private citizen, “there is no general authority to expose the private affairs of individuals without justification in terms of the functions of the Congress,” and “investigations conducted solely for the personal aggrandizement of the investigators or to ‘punish’ those investigated are indefensible.”

With this in mind, the proper tailoring of tax information-related requests by Congress is essential. For example, it may well be that looking at how the IRS audits tax returns of sitting presidents is a worthwhile legislative pursuit; however, assembling all available tax returns of former presidents and arranging the information so that the congressional review does not include ascertaining the identity of the president to whom a given set of tax returns belongs and then ensuring that even this randomized information cannot be publicly disclosed would serve all legitimate legislative needs. Everything else is simple harassment.

To ascribe to Congress greater authority in this area would produce a situation where, under the guise of enacting tax laws, congressional committees could gain access to the tax information of individual Americans, including those regarded by specific members of Congress as political or ideological enemies. This would result in unprecedented abuses of the most sensitive personal information about U.S. citizens that would render Nixon-era IRS abuses tame by comparison.

And, even putting aside partisanship, enabling Congress to snoop on Americans at will is not to be countenanced. What seemingly has eluded Chairman Neal’s supporters is that due process requirements operate with equal vigor on all branches of government, including Congress. Basic due process requirements prevent the executive branch from obtaining private information on U.S. citizens merely because it wants this data.

Instead, when seeking access to financial and other information, law enforcement agencies must demonstrate, usually to a judge, why such information can be legitimately obtained. Improperly gained information is routinely suppressed, and executive branch officials who have obtained it often are reprimanded and even prosecuted. The congressional statute in issue has to be construed with these constitutional imperatives in mind.

There is an additional consideration: Although Congress has oversight authority over the executive branch generally, it has no such authority over the president himself — any more than the president has oversight authority over Congress or the judiciary. Each branch of the federal government is constitutionally equal; none is subordinate. Trump’s business activities before he entered office, and his refusal to make public his tax returns, are not proper subjects of congressional investigation. Although presidential candidates usually release their tax returns as a matter of campaign strategy, Congress could not compel such a release by statute. The Constitution sets qualifications for the presidency, and Congress cannot alter that list.

The fact that Trump’s tax returns are being sought pursuant to a statute that ordinarily would require the Treasury secretary to provide the returns, does not alter the constitutional balance involved. Indeed, the use of Congress’s oversight powers and legislative powers are cabined by the same constitutional principles. The request is based upon an unconstitutional application of a statute — unconstitutional as applied to the situation.

Even if Congress were acting within its constitutional authority, an effort to use its legitimate powers to force disclosure of the president’s tax returns — with the clear goal of debilitating the presidency — would have to be balanced by the courts against the stated congressional need. In balancing otherwise legitimate, but conflicting, assertions of power by the two political branches, courts have looked at their respective needs and the harm that would be inflicted on their respective institutional authorities if one branch were to give way. If Congress does need President Trump’s tax returns for some legitimate legislative purpose, that need will be equally served by providing his returns after he leaves office.

Congress has many powers that can thwart a president’s policy or personnel choices, but only impeachment can personally hold a president responsible for his actions. Even here, it is not clear what relevance a president’s pre-inauguration personal tax returns could have to the question whether he has committed high crimes and misdemeanors while in office.

What Chairman Neal seeks cannot be granted. What is really at stake are not President Trump’s political fortunes but the preservation of the constitutionally required balance of powers between two political branches. Secretary Mnuchin is defending the ability of presidents to function without fear of congressionally driven debilitation. There is every reason to believe that he will prevail in the courts of law as well as in the court of public opinion.

David B. Rivkin Jr. and Lee A. Casey practice appellate and constitutional law in Washington. They served in the White House Counsel’s Office and the Department of Justice under former Presidents Reagan and George H.W. Bush.