No sooner had Senator Ted Cruz of Texas exited the Republican primary on Tuesday night than Hillary Clinton’s partisans on social media began calling for Senator Bernie Sanders of Vermont to follow suit.

Sanders supporters answered those calls with a hashtag that quickly started trending on Twitter: #DropOutHillary.

With his victory in Indiana, Mr. Sanders seems all but certain to remain in the race for at least another month, taking his insurgent presidential campaign to the California primary on June 7. But there is one thing he will definitely leave behind if he ultimately abandons the Democratic nomination fight: an army of online foot soldiers unmatched in size, influence and capabilities, more than ready to join the next battle.

The only question is whether that battle will involve Mrs. Clinton.

According to Personal Democracy Media, which studies the intersection of politics and technology, roughly nine million Sanders supporters have organized through hundreds of Facebook pages, Reddit forums and Slack channels. And Mr. Sanders’s digital corps is not some loose network of supporters informally sharing articles and videos. It is a driving force behind his campaign, soliciting tens of millions of dollars in donations an average of $27 at a time, routinely mobilizing volunteers to perform impressive feats of organizing, and developing cutting-edge technology to aid Mr. Sanders’s run.