Donald Trump's presidential campaign has spent $0 on advertising for the third consecutive month since the primary contest concluded, meaning he is now being outspent by all three of his opponents: Hillary Clinton, Gary Johnson and Jill Stein.

Clinton, the Democratic nominee for president, has continued to funnel money into a negative ad barrage against Trump nationally and in a handful of crucial battleground states. According to the latest ad-spending totals by NBC News, the former secretary of state and her allies have spent $91 million since June.

Libertarian candidate Gary Johnson and Green Party nominee Jill Stein have spent $15,000 and $189,000, respectively, in the same period.

Meanwhile, the National Rifle Association and Rebuilding America Now PAC, which both support Trump, have spent a combined $8.2 million on ads.

Trump has often said he intends to run a lean campaign operation that mostly relies on free media attention. "I don't even need commercials, if you want to know the truth," he said at a campaign rally in June.

But sagging polls numbers and the storm of controversy he created following the GOP convention could force Trump to invest in hard-hitting ads against Clinton in states like Pennsylvania and North Carolina, where the edge he once had has entirely disappeared.

This week's NBC News/SurveyMonkey tracking poll found Clinton leading Trump 51-41 percent among registered U.S. voters.