For the first time since Donald J. Trump was indicted in the spring of 2023, he has lost his grip on the news cycle and — temporarily at least — his message. Instead of commanding morning-to-night media attention, the former president and his allies suddenly find themselves reacting to their opponents.

It’s an unfamiliar experience for Mr. Trump, who has monopolized America’s televisions, newspapers and smartphones for more than 12 months through indictments, primary victories, 34 felony convictions, an assassination attempt and a Republican National Convention at which he was celebrated as a quasi-religious figure.

In the three days since President Biden announced he was quitting the 2024 race, Mr. Trump has entered foreign territory. He has been largely crowded out from “earned media,” or organic news coverage that spreads rapidly among voters and costs campaigns nothing to produce. And his message has been, for the moment, scrambled as Democrats have replaced an old, frail white man with a younger Black woman who is campaigning energetically and giving new life to the Democratic base.

Vice President Kamala Harris, whom Mr. Biden endorsed on Sunday and around whom Democrats rapidly coalesced, has enjoyed a political hot streak that Mr. Biden’s advisers could have only dreamed of during the 2024 campaign.

She has brought in more than $120 million in new donations. She has already drawn bigger crowds than he ever did this election season. She has electrified TikTok and put a jolt into Democrats’ volunteer efforts, especially among Black voters and women. And, unlike Mr. Biden, she is receiving blanket news media coverage that is, so far, overwhelmingly positive.

The Trump team was not unprepared. They had planned for the possibility of Mr. Biden’s dropping out, produced anti-Harris videos and tested her vulnerabilities in private polls. But they were still somewhat surprised when Mr. Biden actually did it. Some of Mr. Trump’s advisers thought he seemed too stubborn — “too Irish,” one aide said — to buckle to the pressure to quit a race against a man he viscerally hated and believed he was best positioned to defeat.