The power of inquiry goes back at least to the 1600s, when Parliament authorized committees to “send for persons and papers” in order to conduct investigations. In other words, it is older than our Republic, and Congress wasted little time in carrying on the British tradition. In 1792, following the defeat of General St. Clair’s army in the Northwest Indian War, the House of Representatives appointed a committee “to inquire” into how such a disaster could have happened and to “call for” all necessary “persons, papers, and records.”

And so it has gone ever since, during both the nineteenth and twentieth centuries. As watchers of “Boardwalk Empire” know, there was a lot of corruption in the 1920s during the administration of President Harding, most notably a bribery scandal involving Attorney General Harry Daugherty and oil reserves in Teapot Dome, Wyoming. Upholding subpoenas of Daugherty’s brother’s bank records, Supreme Court Justice Van Devanter, an extremely conservative and narrow-minded Justice, wrote for a unanimous court that the “power of inquiry” is an “essential and appropriate auxiliary to the legislative function.”

In our time, many Congressional committees have conducted investigations, including Iran-Contra and Whitewater. And the granddaddy of them all, at least until recently, was the Senate Watergate Committee. In 1974, the House was investigating “illegal, improper, or unethical activities” during the 1972 election, and President Nixon resisted the committee’s subpoena of White House tape recordings on the grounds of “executive privilege.” The Court of Appeals for the District of Columbia held that such a privilege was far from absolute and could be overcome if the “public need” so required. And in United States v. Nixon (1974), also a unanimous decision, Chief Justice Burger, a conservative Nixon appointee, wrote that a claim of executive privilege did not override the validity of a subpoena for the White House tapes in the criminal case against Attorney General John Mitchell and several other Nixon henchmen.

Last April, three different Congressional committees, including the House Committee on Oversight and Reform, issued subpoenas to Deutsche Bank, Capital One Financial, and an accounting firm named Mazars USA, all seeking financial records of Donald J. Trump, his companies, and members of his family. The Committees’ stated purpose was to address possible changes to our ethics-in-government laws. Trump sued in the D.C. federal court to prevent compliance with these subpoenas, claiming the investigations served no legitimate purpose.

This case is not to be confused with the New York grand jury’s investigation of the Trump Organization and others arising out of “hush money” paid to two women before the 2016 election. In that case, the Second Circuit Court of Appeals in New York has upheld the Manhattan district attorney’s power to obtain Trump’s financial documents from Mazars USA. The court did not rule on whether Trump is immune from investigation but based its decision on the narrow grounds that the subpoena is directed not to him but to Mazars. Trump is seeking Supreme Court review.

The D.C. case deals with the power of Congress, and the District of Columbia Circuit ruled in October that the congressional committee acted within its power of inquiry. The Supreme Court then gave Trump time to seek review, which he has now done, arguing that the “profoundly serious” constitutional question is whether Congress “can exercise dominion and control over the Office of the President.”

In their 38-page petition, Attorney Jay Sekulow and six other lawyers for Trump correctly point out that Congress is not a law enforcement agency. Its role is to legislate, and it has no right to inquire into purely personal matters including, they say, the President’s finances. So the issue before the Court will be whether Congress is acting within its constitutional authority.

That authority is derived from Article I, which grants various enumerated powers to Congress and then includes a catchall authority to “make all Laws which shall be necessary and proper” to carry out its duties. The first interpretation of those words appeared in McCulloch v. Maryland (1819), where the Court unanimously agreed that Congress had the power to establish a national bank. Chief Justice Marshall wrote, “Let the ends be legitimate … and all means which are … plainly adopted to that end … are constitutional.”

Just to be clear, this is a non-impeachment investigation, not to be confused with the proceedings now before Congress. Nor is it about the motives behind the subpoena which are beside the point, constitutionally speaking. It is, rather, about whether Congress “needs” the information in order to perform its power to investigate and, if need be, to legislate. The specific question is whether the Ethics in Government Act of 1978 needs to be strengthened.  According to the Second Circuit, an individual’s conduct may well offer a “valid point of departure for remedial legislation.”

And it is here that the Congressional subpoena and the impeachment count alleging “Obstruction of Congress” intersect. The Constitution requires the President to “take care that the laws be faithfully executed.” And a President’s obligation to make truthful financial disclosures, as required of every president since Carter, is very much part of our law.

All Supreme Court cases are important. But this one will tell us whether the power of inquiry, and therefore our system of checks and balances, survives.