The week of their debate, Kamala Harris outspent Donald Trump by 20 to 1 on Facebook and Instagram. It was just one sign of how uneven their online advertising battle has become.

Vice President Kamala Harris outspent former President Donald J. Trump by 20 to 1 on Facebook and Instagram in the week surrounding their debate, capitalizing on the moment to plaster battleground states with ads and to hunt for new donors nationwide.

The lopsided spending — $12.2 million to $611,228 on Meta’s platforms, according to company records — was hardly an outlier. Ever since Ms. Harris entered the race, her campaign has overwhelmed the Trump operation with an avalanche of digital advertising, outspending his by tens of millions of dollars and setting off alarm among some Republicans.

Four years ago Mr. Trump, then holding the White House, drastically outspent Democrats online early in the election cycle in hopes of gaining an advantage. Now Mr. Trump, facing a cash shortfall, is making a very different bet that emphasizes the unique appeal of his online brand, the durability of a donor list built over nearly a decade and his belief in the power of television.

The difference was especially stark on screens across the most contested battlegrounds in the week surrounding the debate. In Pennsylvania, Ms. Harris spent $1.3 million on Meta’s platforms, compared with $22,465 by Mr. Trump. In Michigan, she laid out $1.5 million, while he spent only $34,790.

“We just can’t afford to abandon a platform to Democrats,” warned Eric Wilson, a Republican digital strategist and the executive director of the Center for Campaign Innovation, which presses conservatives to embrace technology.

Mr. Trump’s campaign has spent far more heavily on Google, especially on YouTube ads that can closely resemble traditional television. (Google owns YouTube.) But even on Google, a New York Times analysis of advertising records in the seven top battleground states shows that Ms. Harris’s political committees have doubled Mr. Trump’s spending, $25.7 million to $12.8 million, since she joined the race.

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