Mr. Trump’s rallies played a central role to his first victory. With an existential threat to his presidency looming, they are in many ways more important than ever. Rallies let the president “cut through all of the noise” in Washington about impeachment and speak unfiltered to his supporters, said Tim Murtaugh, the Trump-campaign communications director.

“Rallies were always an integral, main part of the campaign strategy, but now it makes communicating with people directly that much more important,” he said. “It’s also important for the president’s supporters to see him out there fighting.”…

Trump rallies are more meticulously produced than the loose and thinly staffed events of four years ago. And while the events don’t earn nearly as much free media as in 2016—cable networks stopped airing them at full length months ago—the campaign has turned them into giant, roving field offices that vacuum up personal data from rallygoers, register new voters and sign up his most enthusiastic supporters as volunteers.

“The rallies,” said Mr. Trump’s campaign chief operating officer, Michael Glassner, “are the driving force behind this movement.”