Sanders' rival had five times as much money, according to new Federal Election Commission filings, beginning the month with $30 million in the bank.
The two were on roughly equal fundraising footing last month, with Clinton and Sanders each raising more than $25 million. But the Vermont senator spent almost $39 million to Clinton's $24 million, the reports showed.
This year, Sanders has averaged more than $40 million in spending per month, underlining how quickly he could blow through the cash he had on hand at the beginning of May, the Associated Press reported on Saturday.
Since he started his presidential bid, Sanders has spent nearly $207 million, about $25 million more than Clinton's $182 million in expenditures. For her part, Clinton has averaged $26 million in spending per month since January.
Sanders' heavy campaign spending wasn't a problem when his online supporters were minting him money. But now that his fundraising has dropped, his high burn rate could hurt his chance to continue competing.
Even as he racked up primary victories last month and sharpened his attacks against the former secretary of state, online donors started holding back. Sanders raised considerably less in April than his record-setting $46 million in March or $43.5 million in February.
The Sanders campaign began taking steps late last month to downsize its operation. He reduced his payroll from about 1,000 to fewer than 400 employees. Sanders has pledged to continue in the race until the final primary, June 14 in Washington, D.C.