NEW YORK: Spending on political TV ads is forecast to drop this year for the first time in decades because Republican presidential candidate Donald Trump has aired far fewer TV ads than Hillary Clinton, his Democratic rival, according to new analysis.

Kantar's Campaign Media Analysis Group (CMAG), the ad-tracking agency, has revised its original July 2015 estimate of $3.3bn advertising expenditure for this year's election and now expects $2.8bn to be spent.

As well as representing $500m less adspend than its original estimate, the revised sum is $300m less than the total spent on TV advertising in the 2012 presidential election.

According to Steve Passwaiter, VP of Kantar Media/CMAG, in an interview with the Wall Street Journal, Republican nominee Mitt Romney and his allied "super PACs" spent $550m on political advertising during that campaign, yet so far Donald Trump and his backers have spent just $78m.

The New York tycoon's campaign did not begin broadcasting TV ads until mid-August and last month spent only $5m on ads compared with the Clinton campaign's $32m.

Yet even Mrs Clinton is spending less on political TV advertising, according to Kantar/CMAG. So far, her campaign has spent $325m on TV ads, compared with the $500m spent by President Barack Obama over the course of his campaign in 2012.

"She probably figures, 'Why do I have to spend more?'," said Passwaiter, given her "overwhelming" advantage on air, the Journal reported.

Donald Trump's campaign recently said it planned to broadcast TV ads worth $100m before Election Day, but it is thought his campaign has not had to dig heavily into its reserves because of the extent of coverage it has been receiving from the broadcast networks.

For example, media-tracking firm mediaQuant has estimated that over the past year Donald Trump has received $4.6bn worth of free media coverage, compared with Hillary Clinton's $2.4bn.

However, despite Kantar/CMAG's downgrading of its overall forecast for this year's election, it has better news for local cable TV operators. It expects local cable TV to see $750m of political advertising, up $150m from the $600m spent in 2012.

Data sourced from Wall Street Journal; additional content by Warc staff