This one always struck me as a weird challenge, and today, per Judge Srinivasan, the D.C. Circuit, sitting en banc, unanimously rejected the argument:
Congress’s choice of a per-election structure thus is not a “prophylaxis-upon-prophylaxis”—a second anti-corruption measure layered on top of the base limits. Instead, the per election structure is an essential ingredient of the base limits themselves—the first layer of prophylaxis. Unlike in McCutcheon, then, there is no warrant for attempting to ascertain whether the per-election timeframe of the $2,600 base limit itself combats corruption. Rather, it is enough if that base limit as a whole (of which its time period is an integral element) prevents the appearance or actuality of corruption in a manner satisfying the closely drawn standard.
I think it is very unlikely that the Supreme Court would want to take this case, which does not challenge the constitutionality of a $2,600 contribution limit itself.