The 1959 contest between Harold Macmillan and Hugh Gaitskell was Britain’s first true television election. First, Labour used the opportunity to drop its opposition to the existence of ITV, then four years old. Second, Labour showcased Gaitskell in a series of effective televised party political broadcasts, a format that was then less than a decade old. (They were overseen by a media-savvy young politician called Tony Benn.) In 2017, the Conservatives are fighting an equally innovative campaign – by getting rid of the voters. Theresa May has taken to speaking in front of carefully chosen crowds, then taking a few questions from selected media outlets, which are chosen in advance.

The temptation is to see this as a non- or a pseudo campaign – as the Prime Minister merely waiting for her healthy poll lead to translate into more seats. But that’s not right. Under the radar, the Conservatives are running a ruthless, effective attack operation. It’s just that many of us will never get to see it.

Why do Facebook and Google dominate the online advertising market? The answer is how much they know about their users – there’s not much value in showing baby milk commercials to the happily childless, after all. Political parties have long had a low-tech version of this strategy: recording party affiliations when out campaigning, so they know which doors to knock on come election day to get out the vote and where to send their leaflets. Now, they can use data on a much grander scale, through Facebook, YouTube, Twitter and the rest.

Read more: Will "dark ads" on Facebook really swing the 2017 general election?

The aim? To send the right messages to the right people. A happy side effect is that the wrong people – voters in rock-solid safe seats, the national media and opposition ­activists – might not see your message at all. Call it a subterranean campaign.

This is important because although targeted messages are powerful, they can backfire. Take Zac Goldsmith’s London mayoral bid in 2016. His team knew that the path to victory involved appealing to white, working-class voters in the suburbs who usually supported Labour, as well as affluent ethnic minorities who had voted for the Conservatives in 2015, allowing the party to extend its hold on seats such as Harrow East and Croydon Central.

In an attempt to appeal to the former group, Goldsmith’s team ran a nasty dog-whistle campaign against Sadiq Khan, who is a Muslim, linking him to Islamic extremists. At the same time, to keep ethnic-minority voters on side, it played up Goldsmith’s support for Narendra Modi, the Indian prime minister. The Goldsmith team also warned that Khan “supports a wealth tax on family jewellery”, a message aimed at those of Indian descent.

These messages were effective with their target audiences, if unpleasant. Goldsmith performed better than Boris Johnson in council wards with large Hindu populations and those with white, working-class voters worried about diversity. But he did significantly worse than his predecessor in wards with large numbers of non-religious voters, liberals and educated professionals. The result was defeat.

Goldsmith’s problem was that his target voters weren’t the only ones who saw the messages he was sending. The jewellery claims appeared on printed leaflets and the accusations of extremism were made in the London Evening Standard. They became part of a wider conversation about whether Goldsmith was running a “racist” campaign – something that clashed with Londoners’ perceptions of themselves and the values of their city.

That’s why the Conservatives’ campaign is so intriguing. Theresa May has a big audience – just not the one in the room. The American strategist Jim Messina, who is working on the May campaign as he did on David Cameron’s in 2015, is fond of saying that the average person thinks about politics for just four minutes a week. These are the people the Conservatives are targeting, and their preferred routes are broadcasters, local newspapers and social media.

On the radio, two broadcasters matter most. The BBC uses the same news clips across its music stations. The other key broadcaster is Global, which owns the bulk of Britain’s most successful commercial stations, including Classic FM and Capital, but most important of all, Heart, which has more listeners than any other private network, including in the crucial battleground of the West Midlands. This strategy has been supplemented by full-page wraparound adverts in local papers – in effect, putting a Conservative poster in every newsagent in every marginal. Critics argue that the design of these does not distinguish them as adverts, but cash-strapped local ­papers cannot afford to be choosy.

Meanwhile, Facebook and other social media websites operate under far looser rules about acceptable content than the broadcasters do, which allows the Tory campaign to run vicious attack videos against Jeremy Corbyn that would never be broadcast on national television.

As a result of all this, the Conservatives are fighting a campaign that is national in name only. Although the central themes – the solidity and strong leadership of Theresa May and the risk of a Corbyn government – are universal, the precise message is tailored to target voters. It also remains safely hidden from the scrutiny of the national media, on citizens’ computers and in their local press.

Labour is also bypassing the conventional media wherever possible and is paying close attention to the pro-Corbyn “alt-left” websites, as well as Facebook pages. However, it is also doing something that might soon be considered rather antiquated: letting Jeremy Corbyn meet voters as he tours the country.