A year later, Mr. Trump has mastered Twitter in a way no candidate for president ever has, unleashing and redefining its power as a tool of political promotion, distraction, score-settling and attack — and turning a 140-character task that other candidates farm out to young staff members into a centerpiece of his campaign.

In the process, he has managed to fulfill a vision, long predicted but slow to materialize, sketched out a decade ago by a handful of digital campaign strategists: a White House candidacy that forgoes costly, conventional methods of political communication and relies instead on the free, urgent and visceral platforms of social media.

“He’s used social media to replace the traditional apparatus of a political campaign,” said Zac Moffatt, who oversaw Mitt Romney’s digital outreach in 2012 and co-founded Targeted Victory, a consulting firm focused on online campaign tactics. “Trump is living on this medium.”

Now, as Mr. Trump enters an uncertain period in his campaign, even rival campaigns acknowledge that Twitter is providing a powerful bulwark against a slide in his poll numbers, by allowing millions of supporters to make his case for him and deflect the controversies he delights in touching off.