CNN  — 

President Donald Trump’s campaign entered October with just $63.1 million in remaining cash reserves, new filings show, underscoring his financial vulnerabilities as Election Day fast approaches.

His Democratic rival, Joe Biden, had nearly three times that amount – more than $177 million – remaining in his war chest, highlighting how the former vice president’s fundraising success in recent months left him with a substantial money advantage as the fall campaign got underway.

Trump’s campaign burned through more money than it took in last month, spending more than $91 million on advertising alone, according to a report it filed Tuesday evening with the Federal Election Commission. But the President’s campaign has been outspent on television in recent weeks, as Biden has battered him on the airwaves, particularly in three swing states – Michigan, Pennsylvania and Wisconsin – that helped Trump secure the White House four years ago.

One sign of the President’s cash crunch: Over the weekend, he flew to deep-blue California for a high-dollar fundraising event that aides say brought in $11 million for his reelection.

Biden outraised Trump in both August and September and together with the Democratic Party committees started October with $432 million remaining in the bank. In all, Trump’s political operation – including his joint fundraising committees with the Republican National Committee – was expected to report $251.4 million in available cash on hand.

But while the President can rely on the larger pool of Republican cash to fund his broader reelection push, including the ground campaign to turn out voters, his main campaign account generally handles advertising. Television stations must offer lower rates to candidates than to other political players, such as super PACs, so candidate money buys more ads.

Trump has trailed Biden in most polling. But Trump campaign officials insist they have sufficient funds to prevail, noting that Democrat Hillary Clinton outraised and outspent Trump in 2016 and still lost the presidency.

“The Trump campaign has all the resources we need going into the home stretch of this election,” campaign spokeswoman Samantha Zager said in an email Tuesday night. “We’re running a comprehensive campaign that incorporates our massive ground game, travel to key states, and ads on digital, TV, and radio.”

Trump campaign manager Bill Stepien this week announced a $55 million ad buy during the final two weeks of the campaign that he described as “more than sufficient air cover.”

Campaign aides have argued that a robust ground operation will pay off for the President on Election Day. Trump officials say the campaign has 2.5 million volunteers who have helped make more than 118 million phone calls to voters in this election cycle.

CNN’s Betsy Klein and Sarah Westwood contributed to this report.