Mr. Rabin-Havt tried to tamp down long-term financial expectations for the Sanders campaign. “It will be remarkable to emulate those 2015 numbers again,” he said.

Separately, Mr. Sanders said during a town hall-style forum on CNN Monday night that he would release 10 years of tax returns “soon,” suggesting he would provide more transparency into his taxes than he did in 2016 when he only released one year of filings. Mr. Sanders said he did not release more returns then because “I didn’t win the nomination.”

“Our tax returns will bore you to death,” he said. “Nothing special about them.”

Money alone is not determinative in presidential politics. In the 2016 Republican primary, a super PAC supporting Jeb Bush burned through $100 million as Mr. Bush finished no better than fourth in any of the first three states. But money does pay for television ads and field staff, and Mr. Sanders’s fund-raising advantage will allow him to take risks and make investments others cannot afford.

“He can build a ground game not just in the first four primary states but into the Super Tuesday states,” Ms. Psaki said. “He can expand his playing field in places like California.”

The Sanders campaign shared new statistics on its donors so far, which skew younger than is typical. An internal analysis showed more than 108,000 of Mr. Sanders’s first day contributors were 39 years old or younger, and they gave a combined $2.5 million of the $5.9 million raised. The most common age for a Sanders contributor was 30. Relatedly, more than $3 million of the donations that first day came from mobile devices, a sign of both shifting habits and the relative youth of his donor base. The campaign did not provide a breakdown by gender.

Mr. Sanders has closely tracked his fund-raising haul, campaign officials said, and Mr. Rabin-Havt said that the senator wrote much of the campaign’s initial 1,400-word email announcement himself.

The email did not explicitly ask for money, instead directing people to sign a petition supporting Mr. Sanders. Of those who signed, about 40 percent also contributed, his campaign said.