Just before 11 a.m. Friday, about seven hours after an overnight social media tantrum from Donald J. Trump, the Twitter account of Hillary Clinton flickered to life.

Mr. Trump’s attacks on a former beauty pageant winner were “unhinged,” Mrs. Clinton said, “even for Trump.” She accused him of valuing women only for their looks. She defended Rosie O’Donnell (“an accomplished actor”) and Kim Kardashian against his “pathetic” insults.

After 20 minutes, Mrs. Clinton concluded with a brief, discordant programming note for the afternoon: “We’ll be in Florida talking about national service,” she said. “You’ll want to watch.”

Limping out of a summer focused almost exclusively on discrediting Mr. Trump, the Clinton campaign has strained in recent weeks to migrate to a more positive message, convening high-minded speeches about Mrs. Clinton’s faith, career and public service aims — only to see Mr. Trump briefly catch her in the polls.