Get ready to see a lot more political ads.

Make America Great Again Inc. — a political action committee — plans to spend $100 million on advertising supporting former President Donald Trump between now and Labor Day, a period spanning the Aug. 19-22 Democratic National Convention in Chicago.

The spending on television and digital ads in swing states such as Michigan and Nevada comes on top of a $37 million ad buy from the Trump campaign over the weekend, according to AdImpact data. The biggest share of the campaign’s spending will focus on Georgia, a state that Democrats swiped in 2020.

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Mr. Trump, the Republican nominee, is trying to pump the brakes on Vice President Kamala Harris’ meteoric rise since taking over the Democratic ticket in place of President Biden last month.

She has drawn large crowds in swing states and is getting media attention over her selection of Minnesota Gov. Tim Walz as her running mate. Polls show Ms. Harris gained territory over Mr. Trump in key states and leads by low single digits in some places.

The Trump campaign is trying to reel voters to its side by defining Ms. Harris as a liberal radical who flip-flops on key issues, failed to secure the southern U.S. border and had a rocky tenure in her home state.

“She’s a San Francisco liberal who destroyed San Francisco, and then as attorney general, she destroyed California,” Mr. Trump said late Monday in a two-hour interview with X owner Elon Musk.

Likewise, MAGA Inc. strategists say their new ads will characterize Ms. Harris as a “soft-on-crime radical who is too dangerous for the White House.”

“Americans might vote for a liberal, but they won’t vote for a lunatic,” PAC strategists David Lee and Chris Grant wrote in a memo, first obtained by Politico.

Alongside the ads, Mr. Trump is increasing his “earned media” — publicity that a candidate gets from third parties without paying them — through interviews, an upcoming rally in Wilkes-Barre, Pennsylvania, and a notable return to posting on X alongside his own social media platform, Truth Social.

Mr. Trump is pivoting back to policy issues after he spent days focused on crowd sizes and attacked podcaster Joe Rogan.

Hoping to preserve its momentum, the Harris campaign this week said it is placing more than 80 billboards in seven swing states. It is the first major ad buy since Ms. Harris selected Mr. Walz as her running mate.

The billboards say Ms. Harris and Mr. Walz are “fighting for you” while Mr. Trump and his running mate, Sen. J.D. Vance of Ohio, are “out for themselves.”

The billboards will appear in English and Spanish and show up in prominent places, including the Las Vegas Strip and interstate highways in Arizona, Georgia, Michigan, North Carolina, Nevada, Pennsylvania and Wisconsin.

“Voters across the battlegrounds are tuning into the clear choice before them: a vision for America that prioritizes the needs of working people, that prizes our rights and freedoms, and that helps all communities get ahead, or a dark vision that drags us backwards and puts billionaires ahead of working families,” DNC Deputy Communications Director Abhi Rahman said.

Some Harris ads amount to an implicit rebuke of Mr. Trump’s “radical” label.

One ad says she took on drug cartels as a prosecutor and will back tough border measures, including the hiring of new agents.

“Fixing the border is tough,” the narrator in a 30-second spot says. “So is Kamala Harris.”

Mr. Trump, speaking to Mr. Musk, said Mr. Biden tasked Ms. Harris to fix illegal immigration, but she failed across the board.

“She never even went there,” Mr. Trump said. “She could have shut the border down.”