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Update: The campaign of Senator Bernie Sanders of Vermont has since revised their numbers: It had raised about $26 million since July and has more than $26 million in cash on hand.

BURLINGTON, Vt. — Senator Bernie Sanders of Vermont raised more than $24 million for his campaign for the Democratic presidential nomination in the last three months, a significant sum that was fueled by a torrid pace of online donations: more than a million so far.

Sanders advisers, in announcing the fund-raising tally on Wednesday night, also said that the senator had more than $25 million in cash on hand.

His leading Democratic opponent, Hillary Rodham Clinton, is expected to announce her fund-raising numbers soon. As of June 30, the close of the second quarter, Mrs. Clinton had $28.9 million in cash on hand compared to $12.2 million for Mr. Sanders.

Mrs. Clinton has been raising money far more aggressively than Mr. Sanders, holding 10 times as many events with donors — many of whom contribute the $2,700 maximum to her primary campaign account.

The latest estimates suggest that Mr. Sanders has spent $15 million on his campaign so far — a large amount, considering that he has yet to make traditionally expensive expenditures on political commercials or polling. Advisers said that most of the spending went to the online fund-raising operation and to hiring staff. Mr. Sanders has 49 paid workers in Iowa, 43 in New Hampshire, 21 at his campaign headquarters in Burlington, Vt., and 27 elsewhere. He also has 15 offices in Iowa and nine in New Hampshire.

The campaign has also paid for a direct-mail program to reach potential donors and supporters and to rent arenas and other large venues in major cities for Mr. Sanders’s large campaign rallies. A full record of his campaign spending will be released by mid-October.

Mr. Sanders’s advisers said they did not have data yet on the proportion of the donations that were $200 or less — a measure of grass-roots support and of his popularity among Americans who are not wealthy. Donations of $200 or less accounted for some 76 percent of the money he raised in the second-quarter campaign finance report.

The onslaught of online donations, collected since May, came at a faster clip than the Internet fund-raising operation of President Obama’s 2008 campaign. Mr. Obama did not pass the mark of a million contributions until the Democratic primaries were under way in the winter of 2008, according to Obama fund-raising solicitations at the time. Mr. Sanders has also amassed 650,000 contributors, well more than Mr. Obama had at the comparable point in that campaign cycle.

Two other points of comparison to Mr. Sanders’s haul of more than $24 million for the third quarter of 2015: Mr. Obama raised $20.2 million for the third quarter of 2007, and Mrs. Clinton raised $23.5 million in that same period, according to the Campaign Finance Institute.

Bernie Sanders Narrows Fund-Raising Gap With Hillary Clinton Mrs. Clinton reported raising more than $28 million in the last three months, only about $2 million more than Mr. Sanders, reflecting a shift in momentum.

Which Presidential Candidates Are Winning the Money Race See how the latest fund-raising numbers from the campaigns and outside groups stack up.

Nicholas Confessore contributed reporting.