Mr. Biden’s support, however, has proved durable, and he has shown himself a challenging politician to attack. The other most frontal assault on him came from Julián Castro, the former federal housing secretary, who also dropped in the polls after their debate-stage confrontation.
In recent months, Ms. Harris had struggled financially as her online fund-raising slowed and her large donors increasingly turned away from her campaign.
And in the days leading up to her withdrawal from the race, Ms. Harris grew increasingly desperate in her search for campaign funds. She surprised one donor who had hosted an event for her but is not a major Democratic bundler by telephoning him to see if he could reach out to his associates who had yet to give in hopes of finding her additional checks for the maximum allowable amount.
On Tuesday, she had a fund-raiser scheduled with some of her top bundlers in New York City that was canceled only hours before the event was set to occur. And on Wednesday, she had been scheduled to attend an event on the other side of the country, in Los Angeles, where Sean Parker, the billionaire tech entrepreneur, was set to host her at his home.
In the third quarter of the year, she spent more than $1.41 for every dollar she raised, burning through millions of her treasury. She stopped buying ads, both online and on television, implemented layoffs, slashed staff in New Hampshire and retrenched to Iowa, where she spent the Thanksgiving holiday with her family.
But it was not enough, as her campaign determined that she did not have the financial resources needed to compete, even as a new allied super PAC began reserving ads on Tuesday. The group quickly began canceling its reservations.
“As the campaign has gone on, it’s become harder and harder to raise the money we need to compete,” Ms. Harris wrote to supporters.