Although the Court split 5-4, it did not divide along traditional ideological lines. Justice Alito wrote for the Court, joined by the Chief Justice and Justices Kennedy, Breyer and Sotomayor. Justices Scalia, Thomas, Kagan and Ginsburg dissented, with all but Kagan writing an opinion. Justice Ginsburg argued the Court’s result was not dictated by existing precedent while Justices Scalia and Thomas reiterated their complaints about existing dormant commerce doctrine (or “negative Commerce Clause” doctrine, as they like to call it). In their view, the Court’s dormant commerce clause jurisprudence is lacks sufficient grounding in the Constitution’s text.
Some commentators (myself included) have claimed the current Court is prepared to relax dormant commerce clause scrutiny, but there’s little evidence such a shift is coming in Wynne. It may still happen, though. Keep an eye on the various dormant commerce challenges to state-level energy and climate policies (e.g. subsidies for in-state renewable power, low-carbon fuel standards, etc.). These cases could split the Court is interesting ways as multiple justices would be presented with a stark conflict between their views of the dormant commerce clause and their views about environmental regulation.