This sentence, perhaps more than any other, in my judgment, drives the decision in Trump v. Anderson (and the unanimous consensus of the Supreme Court):

Because federal officers “‘owe their existence and functions to the united voice of the whole, not of a portion, of the people,’” powers over their election and qualifications must be specifically “delegated to, rather than reserved by, the States.” U. S. Term Limits, Inc. v. Thornton, 514 U. S. 779, 803–804 (1995) (quoting 1 J. Story, Commentaries on the Constitution of the United States §627, p. 435 (3d ed. 1858)).

(This is not without some irony, as Justice Thomas, who joined the majority here, dissented in Term Limits and was critical of Story’s framing.)

From there, the rest of Part II-B opinion flows almost inevitably from this point:

1. States cannot remove sitting federal officeholders under Section 3
2. The Fourteenth Amendment delegates no such power to states even over candidates for federal office
3. The Fourteenth Amendment expressly empowers Congress to enforce it with no corresponding power to states
4. Articles I & II cannot be read to think that they “implicitly authorize” enforcement under the Fourteenth Amendment after the “rebalancing of federal and state power”
5. Congress can lift the disability, & it cannot be burdened by a state imposing the disability before voting begins
6. There is no “tradition” of state enforcement against federal candidates
7. Conflicting procedures in states could yield varying and disparate results in a presidential election

While the last is quite a practical conclusion, the rest are much more text, structure, and history of the Fourteenth Amendment in context. Several of these arguments were echoed in the joint opinion concurring in the judgment by Justices Sotomayor, Kagan, and Jackson (particularly the third and seventh points). The concurring opinion also cites Chiafalo for the proposition that “other constitutional constraint[s]” can limit how states go about administering presidential elections: here, “federalism principles”; there, “Checks on a State’s power to appoint electors, or to impose conditions on an appointment, can theoretically come from anywhere in the Constitution.” And these leave open other questions about how Article II of the Constitution or other provisions about presidential electors could be applied in other, different cases.

But Part II-A of the opinion caused a major breakdown among the justices, so much so that while Justice Barrett was content to concur in part and concur in the judgment by refusing to join this part of the opinion, three other justices broke off to concur only in the judgment while mostly agreeing with Barrett’s position. The contours of that are best left for another post.