As he continues to slip in the polls, Republican presidential nominee Donald Trump is poised to begin airing his first television ads of the general election starting Friday in Florida, Ohio, North Carolina and Pennsylvania, according to a campaign adviser.

The upcoming ad buy was flagged with about 50 members of Congress in a conference call Tuesday afternoon with Mr. Trump’s campaign chairman Paul Manafort and two other staffers. Members were told the campaign would begin buying advertising more widely after Sept. 1.

Mr. Trump, who frequently boasts that he beat several primary challengers who spent far more money than he did, has yet to air a single television ad since he clinched the nomination. Mr. Trump spent only $20 million on ads through May—compared with $62 million by Democrat Hillary Clinton.

In the past two months, Mrs. Clinton has spent about $61 million on general election ads, while pro-Clinton outside groups have spent another $43 million, according to NBC News. She leads Mr. Trump by nearly seven points in national polls, according to the Real Clear Politics polling average, and has posted leads in a number of battleground states.