Those concerns may be warranted. So far, Mr. Trump’s campaign has yet to make reservations for airtime beyond August. Mrs. Clinton’s campaign has reserved time into early September, totaling more $18.1 million, according to Kantar Media. And Priorities USA, the primary group supporting Mrs. Clinton, has put almost $80 million behind ads through the election.

Mr. Trump’s aversion to television spending has been one of the biggest curiosities of his campaign. Mr. Trump waited until a month before voting began in the primary races to start airing television spots, instead banking on the free exposure he had received from seemingly endless interviews. His campaign announced this month that it raised $82 million in July for his campaign and Republican Party committees, leading many in the party to believe an advertising onslaught was imminent.

The Clinton campaign has already spent $55.8 million on television advertising so far this cycle, with $32.3 million of that going to local broadcast television in nine states: Colorado, Florida, Iowa, North Carolina, New Hampshire, Nevada, Ohio, Pennsylvania and Virginia, according to Kantar Media.

It has also been bolstered in those same nine states by $33.5 million in advertising bought by Priorities USA, bringing the total spent in local broadcast television in the nine crucial swing states alone to $65.8 million in support of Mrs. Clinton’s campaign, according to Kantar Media. Priorities USA first went on the air in May, with an attack ad criticizing Mr. Trump’s past misogynistic comments, and have since aired eight additional ads hammering the Republican nominee.

Regardless of the recent additions, the advertising spending is notably less than the 2012 election on both sides. At this point in 2012, President Obama’s re-election campaign had already spent more than $136 million, and Mitt Romney’s campaign $55 million. Restore Our Future, the super PAC supporting Mr. Romney, had also spent an additional $15 million in support by this point, according to Kantar Media.

Not all Republicans see Mr. Trump’s decision to hold back on spending as a mistake.

“There is very little evidence that paid advertising has made any difference in this presidential election to date,” said Neil Newhouse, a Republican who was the lead pollster on Mr. Romney’s 2012 presidential campaign.

Steve Murphy, a Democratic ad-maker and strategist, disagreed, pointing to recent polling.

“Hillary Clinton has a lead in most swing states out of proportion with her national lead, demonstrating the huge success of her advertising campaign,” Mr. Murphy said. “The race should be closer in swing states.”