The Democratic presidential nominee in 2020 will have an excellent chance of winning, so voters would be wise to pay attention even more than usual to the many candidates and their policies. Wasting no time is Elizabeth Warren, who dropped a video to unveil her exploratory committee on the last day of 2018. As the announcement shows, she intends to run as a troubadour for the populist left.

After 30 seconds of home-movie footage, the video takes a grim turn worthy of the Trump inaugural address. She has studied why some Americans...