• The Harris campaign is booking at least $370 million worth of television and online ads in key battlegrounds to run between Labor Day and the November election.
• The spending include $200 million for digital ads on platforms like Hulu, Roku, YouTube, Paramount, Spotify and Pandora.
• Nearly a month since President Joe Biden dropped out of the race and endorsed Vice President Kamala Harris, she has been working to define herself to voters before Donald Trump does so first.

The Harris campaign announced on Saturday that it was booking at least $370 million worth of television and online ads in key battlegrounds to run between Labor Day and the November election.

During the nine-week sprint leading up to Election Day, the Harris campaign is reserving $170 million in TV ads and $200 million for digital ads on platforms like Hulu, Roku, YouTube, Paramount, Spotify and Pandora.

Nearly a month since President Joe Biden dropped out of the race and endorsed Harris, she has been working to define herself to voters before former President Donald Trump does so first.

The Harris campaign's Saturday announcement is an effort to get first pick of ad spots ahead of the Republican presidential campaign.

"By reserving early, the Harris-Walz campaign is securing inventory during high-viewership moments like major sporting events and other national programs before they sell out," Harris' deputy campaign managers Quentin Fulks and Rob Flaherty wrote in a campaign memo.

Those prime-time spots include the season premieres of TV shows like ABC's "Grey's Anatomy" and the "Golden Bachelorette," along with live sports like NFL, WNBA, NBA, NHL and MLB games, the campaign said.

"Rates go up the closer you get to the air date, and there is also less inventory to choose from. So by buying later, Trump is spending more per ad buy and getting worse ad placements, particularly for high-viewership programming like live sports," Fulks and Flaherty added.

The Trump campaign rejected the idea that it needed to play catch-up with Harris and said that her campaign's new ad blitz is a case of overspending.

"Ads supporting President Trump are effectively being seen by more people than [Harris] ads, which proves her campaign is spending recklessly and frivolously because they have no clue on how to run a winning campaign," Trump campaign spokesperson Steven Cheung said in a statement to CNBC.

The Harris campaign's choice to spend $30 million more on digital ads reflects a growing impulse for campaigns to go beyond the traditional TV ad model to reach voters in today's more fractured media environment.

Since Vice President Kamala Harris launched her presidential bid, her campaign has aired over $33 million worth of TV and radio ads, according to tracking firm AdImpact. The Harris Victory Fund has spent over $43 million on Facebook and Google ads, the most of any spender in the presidential race so far.

"We believe we are well on pace to spend more on digital persuasion media than any political organization ever," Fulks and Flaherty wrote in a campaign memo.

The Harris campaign is also placing day-time ads on Fox News in an attempt to reach "a more moderate audience," including supporters of former GOP presidential candidate Nikki Haley, that it believes it can persuade to vote blue.

The Harris campaign's fall ad blitz will come during the crucial last mile of the race when both party's conventions are over and Democrats and Republicans are making their final pitches to swing state voters.

Compared to Biden's 2020 run, the Harris campaign said it is spending double on TV ads in Pennsylvania, more than double in Wisconsin, quadruple in Georgia and six times as much in Nevada.