President Trump’s re-election campaign committee ended September with only $63.1 million in the bank even after canceling some television buys late last month, leaving him badly outmatched financially against Joseph R. Biden Jr. in the final stretch of the campaign.

New filings with the Federal Election Commission showed the extent of Mr. Trump’s cash troubles, which caused the president to fly to California last Sunday for a fund-raiser with just over two weeks until Election Day. The president ended September with just over half as much money as he had at the beginning of the month.

While Mr. Trump’s campaign and its shared committees with the Republican National Committee have raised $1.5 billion since the start of 2019, the disclosures late Tuesday showed that his main re-election committee had only a small slice remaining. That account must pay for many of the race’s most important costs, including television ads.