Bernie Sanders is facing a potentially critical cash shortage as his battle with Hillary Clinton for the Democratic nomination enters its final phase.

The Sanders campaign had less than $6m (£4.1m) in its war chest at the start of May, according to new Federal Election Commission filings, compared to $30m in the Clinton coffers.

The rivals’ fundraising was on roughly equal footing last month, with each raising more than $25m. But the Vermont senator spent almost $39m to Clinton’s $24m, the reports showed.

This year, Sanders’ average monthly spend was more than $40m, underlining how quickly he could blow through the cash he had on hand at the beginning of May.

Since he started his presidential bid, Sanders has spent nearly $207m – about $25m more than Clinton’s $182m expenditure. Clinton has averaged $26m in spending per month since January.

Sanders’s heavy campaign spending was not a problem when his online supporters were donating a lot of money. But now that his fundraising has dropped, his high burn rate could hurt his chances of staying in the race.

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 $46m in March or $43.5m 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, in Washington on 14 June.

Hillary Clinton at a campaign event this week. Photograph: Aaron P Bernstein/Reuters

The latest reports showed that Sanders spent about $21m on media buys and digital consulting. The campaign paid $17.3m to Old Towne Media, based in Alexandria, Virginia, and more than $3m to Revolution Messaging, a Washington advertising firm that concentrates on digital outreach.

Sanders plans to spend a little more than $525,000 on television and radio advertising in California before its 7 June primary, according to advertising tracker Kantar Media’s CMAG. Clinton has not reserved any airtime there.

She has tended to spend less on ads than Sanders. In April, her campaign spent about $9.3m on media buys and $2.7m on online advertising, her report showed.

Sanders reported raising $26.9m in April through his campaign. Unlike Sanders, Clinton has been fundraising for months in partnership with the Democratic National Committee and state parties. Through that joint fundraising account and her campaign, she raised $26.4m in April for her primary battle with Sanders, though fundraising expenses sliced off about $1.4m.

Meanwhile, the DNC and state parties that have benefited from Clinton’s fundraising help have begun investing in likely general election battleground states such as Ohio and Florida.