Details of the campaign's spending, which were due to be reported to the Federal Election Commission by midnight, were not yet available. But the huge outlays came as Sanders was challenging Clinton on various fronts, scoring victories in Colorado, Minnesota and Oklahoma.
For her part, Clinton spent $32.3 million in March while raising $29.3 million. She still ended the month with more money in the bank than her rival: $29 million to his $17 million, according to figures released by the campaigns.
Sanders's online fundraising machine beat Clinton for the third month in a row, and he is close to outraising her for the entire election cycle: he brought in $185.5 million through the end of March from 2.2 million donors, while she collected $191 million from more than 1.1 million contributors.
But Clinton is also flanked by a network of well-funded independent allies. The biggest, the super PAC Priorities USA Action, has amassed $67.2 million so far, including $11.8 million in March.
Priorities' largest donor last month was hedge fund manager James Simons, who gave $3.5 million -- doubling his previous $3.5 million contribution. S. Donald Sussman, another wealthy hedge fund manager, donated $1.5 million, bringing his total contributions to the group to $4 million.
Other million-dollar donors in March were already on The Washington Post's list of top 50 super PAC contributors, including Univision Chairman Haim Saban and his wife Cheryl (who gave $2 million) and Mary Kathryn and Jay Pritzker (who contributed $1.8 million).