WASHINGTON — Senator Bernie Sanders of Vermont raised $18 million in the past three months, his presidential campaign said on Tuesday.
Mr. Sanders’s total was less than the $24.8 million that Mayor Pete Buttigieg of South Bend, Ind., collected in the quarter, an eye-popping sum that his campaign revealed on Monday.
Mr. Sanders was the second candidate to disclose his fund-raising for the second quarter of the year, which went from April through June. After running for president in 2016, he entered the race this time around with a huge network of online donors, giving him a built-in advantage over his opponents in collecting small donations.
But Democratic voters have many more candidates to choose from in this cycle. Mr. Sanders remains far behind former Vice President Joseph R. Biden Jr. in national polling, and he also faces stiff competition from Senator Elizabeth Warren of Massachusetts to win over voters on the party’s left flank.
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Like Ms. Warren, Mr. Sanders is counting on small donations from huge numbers of Democrats to power his campaign. He is not holding high-dollar fund-raising events, though last month he held a grass-roots fund-raiser in San Francisco. In the first quarter of the year, he raised $18.2 million, the most in the Democratic field.
Before the second-quarter deadline on Sunday, Mr. Sanders’s campaign relentlessly reminded supporters that it was raising money very differently than some other campaigns.
In an email to supporters soliciting donations on Saturday, his campaign manager, Faiz Shakir, wrote that some other Democrats were spending the weekend going from fund-raiser to fund-raiser, “holding expensive hors d’oeuvres with one hand while gobbling up fat checks with the other.”
“Some are going to raise ridiculous amounts of money that way,” he wrote. “Obscene amounts. You’ll see it soon.”
Another email later Saturday mentioned Mr. Biden, who is relying heavily on big donors, by name. Mr. Biden, who entered the race in late April, is expected to post a big number for the quarter; he suggested at a fund-raiser last month that his campaign had already brought in about $20 million.
Mr. Buttigieg, too, has vacuumed up big donations from donors on the traditional fund-raising circuit, in addition to bringing in money at grass-roots events and collecting online donations.
The coming fund-raising total for Senator Kamala Harris of California is also expected to be strong, fueled by a combination of traditional fund-raising and online giving. Ms. Harris raised $12 million in the first quarter of the year, more than anyone except Mr. Sanders.
Her campaign said it raised $2 million online in the 24 hours after the start of Thursday’s Democratic debate, when Ms. Harris memorably confronted Mr. Biden over his opposition to school busing, and another $1.2 million online over the weekend.