The Democrats’ carpet-bombing of Starbucks founder Howard Schultz’s third-party bid proves one thing: The party thinks whoever it nominates to run against Donald Trump will be the next president of the United States.

Rather than the distractions of an independent candidate asking what the meaning of “free” is (free health care, free college, free lunch) Democrats want voters focused all the time on Mr. Trump’s bumptious, chaotic personality.

He...