There is now more than a theoretical chance that Hillary Clinton may not be the Democratic nominee for president.

How could that happen, given that her nomination has been considered a sure thing by virtually everyone in the media and in the party itself? Consider the possibilities.

The inevitability behind Mrs. Clinton’s nomination will be in large measure eviscerated if she loses the June 7 California primary to Bernie Sanders. That could well happen.

A recent PPIC poll shows Mrs. Clinton with a 2% lead over Mr. Sanders, and a Fox News survey found the same result. Even a narrow win would give him 250 pledged delegates or more—a significant boost. California is clearly trending to Mr. Sanders, and the experience in recent open primaries has been that the Vermont senator tends to underperform in pre-election surveys and over-perform on primary and caucus days, thanks to the participation of new registrants and young voters.

To this end, data from mid-May show that there were nearly 1.5 million newly registered voters in California since Jan. 1. That includes a 218% increase in Democratic voter registrations compared with the same period in 2012, a strongly encouraging sign for Mr. Sanders.