With a Bernie Sanders-backed progressive facing off against a populist conservative favored by Donald Trump, the Florida gubernatorial race might turn out to be a dry run for the 2020 presidential election. But first, it could set the stage for a long-overdue debate among Democrats.

The Democratic nominee, Tallahassee Mayor Andrew Gillum, advocates Medicare for all, increasing the state corporate tax rate to 7.75% from 5.5%, and boosting the minimum wage to $15 from $8.25. In a statement issued during the primary campaign,...