If Joe Biden falters, the received wisdom runs, nominating Michael Bloomberg to head the Democratic ticket would boost the party’s chances next November by pitting a strong, credible moderate against Donald Trump. It’s possible. But the former mayor of New York could do his party an even bigger favor just by losing.

The reason has to do with the Supreme Court’s 2010 landmark ruling in Citizens United v. Federal Election Commission. Instead of hailing the decision as a welcome victory for the First Amendment—it overturned an...