The idea that the President is not above the law is directly embedded in the Constitution.
Article 1, Section 3 states that even if a President is IMPEACHED and CONVICTED, the President "shall nevertheless be liable and subject to Indictment, Trial, Judgment and Punishment, according to Law."
The Framers made it clear that the law applied to the President just as it did to everyone else
The Supreme Court majority ignored all that, and made the President "a law unto himself"