The advert is in grainy black and white, with an edgy horror movie soundtrack. As gunfights erupt in the streets, the narrator announces in a gravelly bass voice that John Fetterman, Democratic candidate for a US Senate seat in Pennsylvania, “has a love affair with criminals”.

Fetterman has voted “over and over to release the state’s most violent criminals, including murderers”, the narrator says. If elected, he would “keep the drugs flowing, the killers killing, and the children dying”.

The advert was laser-targeted on a demographic which was seminal in securing Joe Biden’s victory in 2020: women over 25 in the suburbs of Philadelphia. That same group could now hold the fate of the Senate in its hands.

Should Philadelphia’s female suburban voters come out for Fetterman on 8 November, they could push him over the winning line in his battle with the Republican nominee, Mehmet Oz. That in turn could help the Democratic party retain control of the upper chamber, and by doing so keep Biden’s agenda alive.

The stakes could not be higher. Yet the Philadelphia women who were bombarded with the “Fetterman loves criminals” ad 6m times over just 10 days through YouTube and Google were told next to nothing about who was behind it.

“Paid for by Citizens for Sanity” is all that the advert reveals in small type at the end of the 30-second video. It took the sleuthing of the non-profit group Open Secrets to expose the producers as former members of Donald Trump’s inner circle, including the far-right senior White House adviser Stephen Miller.

From the other side of the political spectrum comes another grainy black-and-white attack ad, titled Herschel Walker Can’t Be Our Senator. The ad is also targeted exclusively at women, but this time in Georgia, where another nail-bitingly close Senate race is reaching its climax.

“Herschel Walker,” the ad begins, referring to the former NFL star now running as a Republican for a Georgia Senate seat. “Decades of violence against women. Guns. Razor blades. Choking. Stalking.”

The female voters who were besieged by the ad some 60,000 times over four days were only told that it was created by a group named “Georgia Honor”. Open Secrets records that the group is a Super Pac that supports the incumbent Democratic senator, Raphael Warnock, and has so far spent $34m in assailing Walker.

Two grainy black-and-white videos out of a vast mountain of political advertising which is on track this year to smash midterm spending records. It may even exceed the amount poured into the 2020 presidential cycle.

The total investment in 2022 is projected by the non-partisan ad tracking firm AdImpact to be $9.7bn, pushing America close to a stunning new norm: the $10bn election.

Of that, AdImpact estimates that 30% of the political advertising spend, about $2.9bn, is going into digital advertising or to ads placed through connected TV (CTV) – smart TVs that support video content streaming through apps such as Roku or Apple TV.

Such vast sums suggest that the age of the online political ad is firmly upon us. It has been propelled by the “cord-cutting” generation which has dispensed with conventional television in favour of streaming and on-demand formats.

Take Priorities USA, the largest Democratic Super Pac. It has decided to place its entire $30m spend in 2022 in the digital basket – the first time it has entirely dropped broadcast TV advertising.

“Online is where more people are spending their time, especially Black and Latino voters who are critical to the coalition that we are trying to build,” Aneesa McMillan, Priorities’ deputy executive director, told the Guardian. Some 45% of the Super Pac’s spending this cycle has gone on reaching African American and Latino voters, using platform data on social media and YouTube, as well as keywords associated with demographic groups, to target the message.

McMillan said that the shift online was informed by research. The group found that 75% of the TV ads they injected into House races in 2020 went to homes outside the congressional district to be consumed by people who could not even vote in the relevant elections.

The conclusion was clear: “Digital is much more efficient,” she said.

The rise of online political advertising began tentatively with Barack Obama’s first presidential campaign in 2008 and has grown exponentially every cycle since. Despite its billion-dollar size, the world of online political ads remains almost entirely unregulated.

Outside groups, which have beamed millions of attack ads on to voters’ smart TVs and tablets this year, can do so without having to meet federal rules on disclosing who they are or whose money they are spending.

“We live in an increasingly online society, and political campaigns are moving online, but federal transparency rules have never been updated to take that into account,” said Daniel Weiner, head of the elections and government program at the non-partisan Brennan Center.

Adav Noti, legal director of the non-profit Campaign Legal Center, spent 10 years as a lawyer at the Federal Election Commission (FEC) which is responsible for enforcing campaign finance laws. He expressed dismay at the agency’s inability to keep up with a dramatically changing media landscape.

“We are more than a decade into an era of campaigns increasingly being conducted through digital, and the only government agency charged with regulating that activity has done nothing about it. Literally not a single piece of regulation.”

Noti said that one of the effects of the FEC failing to engage with the explosion in online political advertising has been that social media giants and other big digital platforms have been left to their own devices. “Facebook, Google, TikTok and the rest have become the de facto regulators, and they set their own rules.”

The big players have gone in different directions. Facebook and Google have both set up public databases listing their political ads, introducing a modicum of transparency.

Other platforms such as TikTok have prohibited political advertising, though candidates are increasingly using the sites directly as megaphones.

Attempts by Congress to legislate for more accountability have all succumbed on the rock of Republican intransigence in the US Senate. The Honest Ads Act, a bipartisan bill backed by the Brennan Center that would make digital ads subject to the same disclosure rules as broadcast TV and radio, was included in the Freedom to Vote Act that failed to overcome a Republican filibuster in January.

In the absence of central regulation, outside groups can distribute extreme or false messages with impunity. Citizens for Sanity, the Super Pac created by former Trump advisers, blasted out an advert last month attacking Biden’s immigration policy.

It was viewed 600,000 times over nine days by voters in the border state of Arizona.

“Who is Joe Biden letting in?” its female narrator asks. “Joe Biden and his fellow Democrats have erased our southern border and released a record number of illegal immigrants into the United States, all at your expense.”

The ad goes on to warn about a “giant flood of illegal immigration” that was “threatening your family”. It accuses Biden of allowing drug dealers, sex traffickers and violent predators into the country, one of whom raped a little girl.

“She was three years old,” the narrator says.

The Poynter Institute’s factchecking unit, Politifact, reviewed the ad. It found that the immigrant who allegedly sexually assaulted a three-year old girl had been in the US since at least 2011; he has been behind bars since February 2020 – almost a year before Biden entered the White House.

Politifact rated the advert “False”.