The Vermont revolutionary’s victory portends a long primary battle, unless Democratic voters elevate a single mainstream candidate who can challenge him. Mr. Sanders will get his 25% to 30% primary after primary, racking up delegates on his way to the convention. If other candidates keep dividing the other votes, he will be hard to stop, as Mr. Trump was for Republicans in 2016. Even if a single alternative emerges, Mr. Sanders won’t go down without a ferocious intra-party fight.

How did this happen? How did Mr. Sanders move from the socialist fringe to the brink of controlling the Democratic Party? The Senator’s dogged persistence across decades and especially the last four years is part of the explanation.

Yet Mr. Sanders wouldn’t be this close to the White House if not for the complicity of Democrats and the liberals who dominate the academy and media. Rather than fighting the ideas that animate him and his millennial voters, they have indulged and promoted them. They created the political environment in which he could prosper. Consider the intellectual currents he is riding…