So much for triangulating. After Bernie Sanders suspended his presidential run last week, Joe Biden waited barely 24 hours before racing to bolster his progressive bona fides. Mr. Biden said Thursday he plans to make 60-year-olds eligible for Medicare, while erasing undergraduate student debt for middle-class borrowers.

For months, Mr. Biden has opposed Medicare for All, saying a better path would be to “build on” ObamaCare with a “public option.” If he really believed this, he wouldn’t deprive his public option of 20 million...