For longtime Clinton allies, the elevation of Mr. Bannon has inspired competing emotions. There is a sense of vindication, bordering on the surreal — a we-told-you-so impulse that cannot be suppressed as purveyors of conspiracy theories seize the reins of an actual Republican presidential campaign.

“She was right, that there were these dark forces at work that were trained on her and her husband,” said Lissa Muscatine, a friend of Mrs. Clinton’s and longtime chief speechwriter. “The extent of it didn’t really add up for people at the time, but maybe now it does.”

Yet after more than two decades of attacks from conservatives — some seizing on Mrs. Clinton’s own missteps, her former aides say, but most generally groundless — others worry that an even darker turn is possible, given the advisers now guiding Mr. Trump’s campaign…

“This is beyond anything anyone even thought about in their worst nightmare,” Mickey Kantor, a longtime Clinton family friend who was the United States trade representative and secretary of commerce under Mr. Clinton, said of the new conservative coterie around Mr. Trump…

“She can dismiss Fox News or Breitbart, but this is her opponent now,” said Patti Solis Doyle, who worked for Mrs. Clinton from 1991 to 2008. “They’re just going to have to weaponize, contrast with him, respond to attacks.”