Mr. Biden has maintained a nearly 2-to-1 advantage on the airwaves for months. His dominance is most pronounced in three critical swing states — Michigan, Pennsylvania and Wisconsin — where he spent about $53 million to Mr. Trump’s $17 million over the past month largely on ads assailing the president’s handling of the virus as well as the economy and taxes, according to data from Advertising Analytics, an ad tracking firm.

In Pennsylvania alone, Mr. Biden ran 38 different ads during a single week this month, a sign of how comprehensive his effort there has been.

The president’s ad strategy, in turn, reflects the challenges facing both his campaign finances and Electoral College map. He has recently scaled back advertising in battleground states like Ohio and Iowa and, until this past week, slashed ads in Michigan and Wisconsin, despite being behind in polls. And Mr. Trump is having to divert resources to hold onto Republican-leaning states like Arizona and Georgia.