Hyper-visible on the campaign trail, US President Donald Trump has been far more discreet in on-air advertising than his rival Joe Biden in a presidential race that is shattering ad spending records.

Barack Obama's former vice president is projected to spend twice as much as the Republican billionaire incumbent on television advertisements by election day November 3, according to data compiled by firm Advertising Analytics.

Even more striking, since September 1 the group has seen team Trump "completely reduce... or reduce greatly" its broadcast ad funding in several Midwestern states and other battlegrounds like Pennsylvania that could determine his fate in the election, said Advertising Analytics vice president John Link.

It is thanks to wins in several of these states that Trump secured his shock 2016 victory over Hillary Clinton -- and where the president is now struggling in polls against Democrat Biden.

In bellwether Ohio, with the race on a knife edge, the president committed $7.8 million in September broadcast advertising, but ended scrapping all but $302,000 of that advertising, Analytics data shows. 

Biden for his part committed $1.3 million to Ohio ads in September, and spent it all.

Trump's team no doubt has reinvested part of that spending to other states in the southern "Sun Belt," most notably Florida, an absolutely crucial state in the White House race, and Georgia.

But the Trump campaign's "overall spend on a weekly basis is being reduced," Link said. 

Biden by contrast has expanded the map of ad spending to states that are not traditionally in play, such as Georgia, which has not voted Democratic in the presidential election since 1992. The state is rated a toss-up by RealClearPolitics.

Is Trump's gambit clever because he feels he does not need to spend so much in these states, as he asserts, or is it a sign that his war chest is depleted?

Either way it is a "risky strategy," Link said.

Airing television spots, particularly on broadcast networks, "is the strongest vehicle for impactful ad messaging," Link explained, because it allows campaigns to reach that often elusive but always courted demographic: undecided voters.

The proof is in the spending: the campaigns and groups that support them will not have pumped a historic amount of money -- projected by Ad Analytics at between $2.75 and $2.8 billion by November 3 -- into advertising if it didn't work.

Such astounding numbers include the record-breaking spending during the Democratic primary, which saw billionaire Michael Bloomberg spend lavishly to fund his own doomed campaign.

 - 'Winning strategy'? -

 Trump's campaign admits it is using "strategic, surgical" ad spots instead of carpet-bombing media markets.

"It makes no sense to run TV ads in states we know we're going to win," Samantha Zager, a spokesman for Trump's campaign, told AFP.

Instead the Republican camp is relying on a precise database of American voters to target its ad efforts, Zager said.

She highlighted how Trump spent less than Clinton did four years ago, and still won. 

"Maybe it's time for the mainstream media to accept our winning strategy and start questioning why Joe Biden is needlessly overspending on TV," Zager said.

Things however are different this year. The tempestuous Republican is not benefiting from an avalanche of free coverage he received in 2016.

The coronavirus pandemic, and Trump's time off the trail as he recovered from Covid-19, has led to fewer rallies and less wall-to-wall campaign coverage.

Even if the perpetual showman intensifies his schedule in the home stretch, networks have dedicated far less time to his rallies than in 2016, when lengthy broadcasts of events -- including when stages were empty -- were not uncommon as media heaped attention on the new Trump phenomenon.

 - Biden's deep pockets -

The financial status of each candidate helps explain their advertising investments: Biden has just completed two months of record fundraising and had $432 million in the bank in late September, versus $251 million for Trump.

But the president's campaign, in collaboration with the Republican Party, intends to surge advertising investment by 40 percent in the last weeks compared to its planned budget.

Advertising remains vital, stressed Chris Jackson of research group Ipsos.

"Not because it's going to change minds, but because you do still have to get your people to go out and vote," Jackson told AFP.

And during such a "chaotic" year upended by the pandemic, rallying the faithful to the voting booth through advertising is even more important.

"If Biden can turn out his voters and things don't change" in the polls, Jackson said, "he's in a good position to potentially win the election."