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“There is zero question now that Bernie will be able to build a very strong and formidable campaign operation,” said Mo Elleithee, who was a spokesman for Hillary Clinton’s 2008 presidential campaign and is now the executive director of Georgetown University’s Institute of Politics and Public Service.

But Mr. Elleithee said it remained to be seen whether Mr. Sanders’s backers would stick with him as time went on. “What will be interesting is: Is this sustainable as other candidates get better known?” he said.

Mr. Sanders posted a bigger opening number than he did in the last presidential race, when he raised about $14 million in the quarter that covered April through June of 2015. Mr. Shakir said it took 146 days, or about 21 weeks, for Mr. Sanders to reach 900,000 donations in that campaign.

But the amount raised by Mr. Sanders this quarter was not off the charts for a Democratic presidential hopeful. Mrs. Clinton raised about $47 million in her first quarter as a candidate in the 2016 campaign, though unlike Mr. Sanders, she relied heavily on big donations.

Mr. Sanders began the 2020 race with the benefit of an enormous donor base that he developed in his primary campaign against Mrs. Clinton, when he raised well over $200 million. After jumping into the 2020 race on Feb. 19, he quickly reminded his rivals of his large following, collecting $5.9 million in the first 24 hours and a total of $10 million within a week. (Only Mr. O’Rourke disclosed a bigger 24-hour haul, with $6.1 million.)

The Sanders campaign had set a goal of receiving one million donations by the end of the first quarter. The campaign fell short of that goal, but Mr. Shakir expressed no regret about having aimed for it. “It was important to us to set an ambitious goal,” he said.

Mr. Sanders’s financial resources go beyond the money he has received since entering the presidential race. Like others running for president who have previously run for federal office, he can make use of funds that he had raised in earlier campaigns. He began the 2020 race with $14 million available for his use, Mr. Shakir said.