Perhaps one aspect of the case will end up framing the whole. Aggregate contribution limits do not affect independent spending, and they do not stop any individual from expressing a modicum of support for all 535 members of Congress, or indeed, every single one of those representatives’ challengers. There is little reason to believe that aggregate limits systematically favor the electoral fortunes of incumbents, or make it difficult for any individual candidate to run a campaign. Their most notable impact seems to fall on political parties with multiple competitive campaigns to feed (and party-wide SuperPAC ads to refute).
But these parties rarely need the Court to save them from the legislature. They are already amply represented in Congress, and could lawfully repeal the limits at any time. Instead, they have – at least for now — tied themselves to the mast. It is possible that that image will be the one the Justices end up finding most compelling.