SSP EW Scripps Company

Stocks across the broadcast industry fell Tuesday as investors feared that television advertising spending on the race for the White House wouldn't be as high as expected.

Sinclair Broadcast Group Inc. withdrew its political-spending forecast for the year on Tuesday morning because of the "unusual nature" of this year's election. As recently as August, the company said it was expecting a "record year" for campaign spending.

Shares of Sinclair fell 9.9% to $25.84 in afternoon trading. E.W. Scripps Co. shares dropped 8.8%, Tegna Inc. declined 4.4%, and Tribune Media Co. fell 4%.

Over all, political ad-spending levels for this presidential election have shrunk from 2012, as Republican nominee Donald Trump has aired an historically low number of TV ads. In 2012, Republican nominee Mitt Romney spent nearly $50 million on advertising through July. Mr. Trump spent $20 million in the same period this cycle, according to the latest figures disclosed to the Federal Election Commission.

Mr. Trump's $20 million was spent entirely on the primary. After becoming his party's effective nominee in May and officially accepting the nomination in July, his campaign didn't begin airing TV ads again until mid-August. Since mid-June, the Clinton campaign has aired nearly 115,000 TV ad spots, compared with 20,000 aired by the Trump campaign, according to the Center for Public Integrity.

Industry watchers are divided over whether the lower spending is a short-term anomaly caused by the abnormal nature of Mr. Trump's campaign or a sign of a broader rollback of the importance of television advertising.

Throughout the primary, Mr. Trump relied largely on free media through interviews with cable networks, which often carried his rallies live on air.

"It seems to be more of a Trump story than a general story," Travis Ridout, a Washington State Professor who wrote a book about campaign advertising, said. He added that in "this presidential campaign, everyone knows too much about Hillary Clinton and Donald Trump, and thus the ability for advertising to have any impact is really quite small."

Mrs. Clinton's ad-spending levels this election cycle are roughly consistent with what President Barack Obama spent in 2012. Through July of that year, he spent $111 million on advertising, compared with $108 million spent by Mrs. Clinton in the comparable period this year.

"While we previously anticipated a decline in presidential ad spending in the third quarter based on the late fundraising by the Trump campaign, we have yet to see significant spending, even at the levels we initially anticipated," Sinclair's Chief Financial Officer Chris Ripley said.

For the third quarter, Sinclair now expects to take in $46 million of political spending, down from its previous estimate of $58 million to $68 million.

Sinclair is exposed to a number of battleground states, including having 20 or more channels each in Ohio, Florida, Pennsylvania and Michigan and markets in North Carolina and Virginia. In 2012, total political spending accounted for 11% of its annual broadcast revenue.

Still Sinclair said it was "hopeful" presidential advertising spending will increase as polls tighten.

Jan Dawson, Chief Analyst of Jackdaw Research, estimates that presidential campaigns account for about 30% of total political spending for local TV station owners.

Mr. Trump's super PAC firepower on the airwaves looks likely to improve in the coming weeks, as two billionaires—casino owner Sheldon Adelson and TD Ameritrade founder Joe Ricketts—have pledged a combined $6 million to support the nominee. Their efforts will at least in part be aimed at erasing Mrs. Clinton's major advantage in TV advertising.

Mr. Trump and his allies are likely to ramp up their spending on TV ads in the final stage of the election. Since returning to the airwaves in mid-August, the campaign's ad spending has escalated each week. In the week beginning Sept. 4, his campaign aired nearly 10,000 ad spots, up from 6,000 the previous week.

Mr. Trump's campaign has said it doesn't see the need to spend as much on TV ads as Mrs. Clinton; the Trump camp has said it is looking to run a leaner campaign operation.

Write to Austen Hufford at austen.hufford@wsj.com and Rebecca Ballhaus at Rebecca.Ballhaus@wsj.com

(END) Dow Jones Newswires

September 20, 2016 15:25 ET (19:25 GMT)

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