It’s finally happening. Bernie Sanders just announced that he’ll be running for president again.

Judging from its early entrants, the politics of this upcoming Democratic presidential primary will be the most progressive in decades. All of the confirmed contenders claim to support Medicare for All, for example — an idea that most in the party ignored or outright rejected as recently as three years ago. Some candidates are also running on ideas like universal child care and a Green New Deal, a notable departure from the pro-corporate politics that have characterized the party’s program for years.

That leftward shift is a positive development, and there’s a reason for it. The reason is Bernie Sanders.