The opinion wasn't directed at Nixon, it was directed at William Jefferson Clinton.
The irony of all this is that if you dig deep enough into this legal question, you may very well end up concluding that most of the U.S. Criminal Code is completely unconstitutional.
Where do you find that in Article III? Sure sounds to me like the Framers were creating a Federal criminal trial system.
Section 2The judicial Power shall extend to all Cases, in Law and Equity, arising under this Constitution, the Laws of the United States, and Treaties made, or which shall be made, under their Authority;—to all Cases affecting Ambassadors, other public Ministers and Consuls;—to all Cases of admiralty and maritime Jurisdiction;—to Controversies to which the United States shall be a Party;—to Controversies between two or more States;—between a State and Citizens of another State,—between Citizens of different States,—between Citizens of the same State claiming Lands under Grants of different States, and between a State, or the Citizens thereof, and foreign States, Citizens or Subjects.
In all Cases affecting Ambassadors, other public Ministers and Consuls, and those in which a State shall be Party, the supreme Court shall have original Jurisdiction. In all the other Cases before mentioned, the supreme Court shall have appellate Jurisdiction, both as to Law and Fact, with such Exceptions, and under such Regulations as the Congress shall make.
The Trial of all Crimes, except in Cases of Impeachment, shall be by Jury; and such Trial shall be held in the State where the said Crimes shall have been committed;
Just one random example is 18 U.S. Code § 36 ... which is titled Drive-by shooting
What the hell is the constitutional basis for the Federal government to be prosecuting crimes for drive-by shootings?
P.S. — That’s why the U.S. Department of Justice didn’t even exist before 1870. The duties of the U.S. Attorney and his District Attorneys were primarily civil in nature. Treason and tax evasion were probably the only notable crimes that were tried in Federal courts.