You’ve heard of single-issue voters, but how about a single-issue candidate? Jay Inslee, the Democratic Governor of Washington state, said Friday he’s running for President “because I’m the only candidate who will make defeating climate change our nation’s No. 1 priority.” As a sign of seriousness, the announcement video ends with a cameo from Bill Nye.

Mr. Inslee’s official website doesn’t have an issues page, only a proposal for a “Climate Mission.” It’s described as a “10-year mobilization” to “accelerate the transition...