On her final business day as Federal Trade Commission chair, Lina Khan—with the Democratic majority—voted Friday to sue PepsiCo Inc. for allegedly giving a big-box retailer unfair pricing advantages in violation of a law little-used in the last two decades.

But like a Pepsi can shaken then opened, Republican FTC Commissioner Melissa Holyoak exploded at the majority's legal reasoning, calling it "the worst case I have seen in my time at the Commission."