The full strength of these activists — many of whom are not old enough to vote — did not become clear until last fall, when they were key to one of the year’s most surprising upsets, helping Sen. Edward Markey defeat a primary challenge from Rep. Joe Kennedy III, who had been heavily favored to win. In conversation, Walsh tends to downplay her movement, describing them as “Markey teens” and “theater kids” who “formerly ran, like, Taylor Swift or K-pop stan accounts.” But the Markeyverse carried out a devastating political maneuver, firmly fixing the idea of Markey as a left-wing icon and Kennedy as challenging him from the right. They carried out ambitious digital organizing, using social media to conjure up an in-person workforce — “an army of 16-year-olds,” as one political veteran put it, who can “do anything on the internet.” They are viewed apprehensively by many in Massachusetts’ Democratic establishment, who say that they smear their opponents and are never held accountable; that they turn on their allies at the first whiff of a scandal; and that they are attacking Democrats in a coordinated effort to push the whole party to the left, much as the Tea Party did, on the right, to the Republicans.