The decision is a great relief because a contrary decision, as I explained last September at Slate, could have led to great chaos (a point Rick Pildes explained in more detail here). The opinion for 7 Justices by Justice Kagan relies a great deal on history and the strong historical power of states to set the rules on choosing presidential electors. It is a typically breezy and readable Kagan opinion, compete with references to Veep and “Hamilton.”
I understand that the motivation of Larry Lessig and his compatriots to bring this case was to try to create the conditions to blow up the electoral college and lead to a national popular vote for President. As the Slate piece explained, this is not the way to do that, much as I am no fan of the college. And I think the National Popular Vote plan is dangerous as well, for reasons Ned Foley explains in his new book. The way to get rid of the Electoral College is through constitutional amendment, something not likely to happen for a while. (I would suggest a constitutional right to vote is of more pressing urgency and could pave the way for electoral college reform as well).
A few more points of note about the decision. First, in footnote 4 of the Court’s opinion, the Court casts serious doubt on laws that would require presidential candidates to submit tax returns as a condition to running for office. (It also suggests that racially discriminatory practices in choosing presidential electors would be unconstitutional).
Second, the Court included an escape hatch to these rules in the event that a presidential candidate dies between election day and the date for the electors to vote.
Third, the Court once again declined to cite its 2000 decision in Bush v. Gore despite the fact that the case was directly on point. Only once in a Justice Thomas concurrence has any member of the Court cited that case for any reason.
Finally, as Steve Mazie notes, part of the reason for the unanimity (though not on the reasoning, with J. Thomas, joined in part by J. Gorsuch, getting there on a different path) is the lack of a clear ideological or partisan winner on this question. On questions with such valence, 5-4 is much more likely.
(More about the case from Derek Muller.)