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“A nationwide network of hundreds of thousands of grass-roots supporters has stepped up to lay the foundation for a winning campaign,” said Juan Rodriguez, Ms. Harris’s campaign manager, in a statement.

Ms. Harris has also raised money aggressively on the traditional fund-raising circuit and spent the final weekend of March dashing across California, from the Bay Area to Los Angeles, to raise as much as possible ahead of the first disclosure deadline of the 2020 race. Her campaign said only 0.55 percent of her individual donors gave the legal maximum of $2,800 — though that is still hundreds of contributors.

Earlier on Monday, Pete Buttigieg, the mayor of South Bend, Ind., announced that he had raised more than $7 million in the first quarter. Mr. Buttigieg’s campaign said he had 158,550 donors who gave an average donation of $36.35. About 64 percent of his $7 million came from contributions of less than $200.

Neither Ms. Harris nor Mr. Buttigieg disclosed how much cash on hand their campaigns currently have.

On Tuesday, senior advisers to Senator Bernie Sanders, who is widely expected to post the largest fund-raising haul this month, are scheduled to release his fund-raising figures. In emails to supporters, Mr. Sanders’s campaign has said he had nearly 900,000 contributions in the first quarter, writing in one email that donations were “about a $20 average this time.” His average donation was $27 in 2016, suggesting he has raised somewhere between $18 million and $24 million.