ENLARGE Republican presidential candidate Donald Trump, shown in a July 6 file photo. Photo: Associated Press

Nearly half of voters in a recent survey said they had seen TV ads supporting Donald Trump in the last week.

There’s just one problem: His campaign hasn’t aired any, and his friendly super PACs have run very few.

Meanwhile, 52% of respondents said they had seen an ad promoting Democrat Hillary Clinton—just six percentage points more than the proportion who said they had spotted Trump ads. Mrs. Clinton has aired more than 20,000 TV ad spots since June 8, on top of more than 11,000 aired by her friendly super PAC, according to the Center for Public Integrity, which analyzed Kantar Media/CMAG ad data.

The survey, conducted by the digital marketing firm Fluent, illustrates the challenge that Mr. Trump has long posed for his rivals: He may not be running ads, but he is always on TV.

“I have a little TV in my office. If I put it on right now, I have a 75% chance of seeing him,” said Jordan Cohen, the chief marketing officer at Fluent. He attributed survey respondents’ belief that they had watched a Trump ad to Mr. Trump’s ubiquity on TV and social media.

Indeed, Mr. Trump has mastered the art of getting on TV without paying for it. His rallies are frequently carried live by cable networks. He often calls into the networks for interviews. And networks regularly splash his most controversial tweets across the screen for pundits to dissect.

The media-coverage tracker mediaQuant estimated that Mr. Trump has received more than $3.8 billion in such unpaid media exposure in the last year, compared to $1.7 billion for Mrs. Clinton.

To be sure, while Mr. Trump’s campaign hasn’t run any ads in recent weeks, some super PACs have aired spots on his behalf. But those buys have been relatively small—certainly not enough to be seen by 46% of poll respondents. Great America PAC, the most active pro-Trump group so far, has not aired any ads on broadcast or national cable TV, but has run ads on satellite and local cable TV.

It remains to be seen whether Mr. Trump can get away with airing so few ads compared with Mrs. Clinton. Through the primaries, Mr. Trump spent $20 million on ads through May—compared with $62 million for Mrs. Clinton.

While Mr. Trump’s fundraising had lagged in recent months, he said last week that he had raised $20 million for his campaign, on top of about $32 million for two joint funds with the Republican National Committee—a fraction of which will be transferred to his campaign. That was a substantial increase from May, when he raised just $3.1 million. That burst of fundraising could translate to more TV ads in the coming weeks.

The Fluent poll surveyed 1,248 registered voters online on June 2, and has a margin of error of plus-or-minus 2.8 percentage points.

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