The oath makes only scant appearances in Supreme Court case law, and figures not at all in most of the seminal cases involving the scope of presidential power.
But don’t let that fool you. It is actually a fundamental underpinning not only of many of our cultural assumptions regarding presidential behavior, but also of the judiciary’s willingness to grant deference to the executive branch, particularly in the realm of national security. This point is actually not limited to the presidency. Oaths of office are, more generally, a foundational component of inter-branch deference and comity. When the courts presume a statute constitutional, they do so because it was passed by a coordinate branch of government whose members swore oaths to protect the Constitution. The presumption of constitutionality is, in other words, a deference to the institution of a coordinate branch as populated by people who swear oaths to the Constitution. Similarly, when the courts presume regularity in executive behavior or defer to the executive branch on a matter entrusted to it, the source of the presumption is that the office-holder has sworn an oath too.
To understand the relationship between the oath and the deference to the president in national security matters, start with Joseph Story. In Commentaries on the Constitution, Story wrote of the oath of office generally:
Oaths have a solemn obligation upon the minds of all reflecting men.... If, in the ordinary administration of justice in cases of private rights, or personal claims, oaths are required of those, who try, as well as of those, who give testimony, to guard against malice, falsehood, and evasion, surely like guards ought to be interposed in the administration of high public trusts, and especially in such, as may concern the welfare and safety of the whole community.Of the presidential oath particularly he said:
No man can well doubt the propriety of placing a president of the United States under the most solemn obligations to preserve, protect, and defend the constitution. It is a suitable pledge of his fidelity and responsibility to his country; and creates upon his conscience a deep sense of duty, by an appeal at once in the presence of God and man to the most sacred and solemn sanctions, which can operate upon the human mind.
Once a government official, and particularly a president, swears the oath of office, in other words, we assume that the oath binds the person to a certain degree of public virtue and good faith, along with inculcating a sense of duty and responsibility. That is something we can defer to.
There’s even a school of thought that the oath of office is a critical foundation for inherent presidential powers. Dissenting in Youngstown, Chief Justice Fred Vinson cites the Presidential Oath Clause in conjunction with Alexander Hamilton’s description of “Energy in the Executive” in Federalist 70 as evidence that “the Presidency was deliberately fashioned as an office of power and independence”—which, he argues, grants the president expansive powers in a time of emergency. Likewise, the Court in U.S. vs. U.S. District Court (Keith) notes that “implicit in that duty [to preserve, protect, and defend the Constitution under the oath of office] is the power to protect our Government against those who would subvert or overthrow it by unlawful means.” And according to Justice Lucius Lamar’s dissent in In re Neagle—another case involving questions of inherent presidential powers—the government argued before the Court that the presidential oath “by necessary implication … invest[s] the President with self-executing powers … independent of statute.” (Unlike his brethren, Lamar did not find this argument convincing.)