California gave birth to the so-called gig economy with startups like Uber, Lyft and Postmates that are used by millions. But now Golden State Democrats are trying to smother their babies, and the progressives running for President want to help.

Gig companies rely on flexible workforces to meet consumer demand—whether pizza delivery at midnight or a plumber for a weekend leak. Most workers use these gigs to supplement other forms of income, and they like to set their own hours and toggle between platforms.

Unions...