Data mining, dark ads, fake news, Russian bots… our voting system is acutely vulnerable to shady influences because of its fixation on marginal constituencies.

We’re all hearing a lot these days about sinister forces attempting to hijack elections. Whether it’s Russian social media accounts posing as your neighbours or unscrupulous tech companies using your data to play on your deepest fears, there’s a sense that democratic processes are coming under attack.

Should we be worried? Influencing a general election sounds like a difficult thing to do. 32 million people voted in 2017. Surely these sort of tactics can’t change that many minds?

But because of our archaic voting system, they don’t have to. First Past the Post offers illicit online influencers, and even foreign powers, the ideal environment in which to influence our ballots.

In UK general elections, it’s only a tiny minority of swing voters in marginal constituencies that decide the outcome. For example, the Conservative Party would have won an overall majority last year if just 533 people in 9 particular constituencies had voted differently, whilst Jeremy Corbyn would have won a majority if just 0.16% of voters (51,072) had opted for his party over the Conservatives. 31 constituencies were won by a margin of fewer than 500 votes.

Politicians and parties know this. For years they’ve tailored their messaging and resources to target marginal constituencies. According to the Electoral Commission, 37.5% more on average was spent campaigning in marginal seats than in safe seats last year.

And it works. As long as we have First Past the Post, any party that is serious about winning is obliged to pile resources into the few seats that might change hands, to the neglect of the majority which almost certainly will not.

The problem is, these new threats – data harvesting, targeted ads, Russian bots and fake news – exploit exactly the same weakness in our democratic system. They don’t need to change the minds of millions of people to swing an election: they just need to influence thousands or even hundreds in the right places.

The tools are all there. Advertising on social media is already able to target key demographics in particular geographical areas with minute precision. Add to this industrial levels of data mining, fake news outlets tailoring scare stories to play on the fears of these audiences, advertising by unknown and unaccountable sources, bot accounts spreading and intensifying these messages, and things start to get unsettling.

With a system of Proportional Representation – in which every vote counts – it takes millions of votes to alter the final result. In Germany’s Federal Election last November, Merkel’s CDU came out on top – winning 93 more seats than the Social Democrats. If the Social Democrats – or anyone else – wanted to reverse this outcome, there would be only one way to do it: persuade 2.9 million voters to switch allegiance.

Because seats match votes in Germany, influencing an election outcome means persuading millions of voters. In the UK, just 533 people voting differently could have meant a Conservative majority instead of a hung parliament.

When seats match votes, to win a lot more seats you need to get a lot more votes.

Not so with First Past the Post – or other winner-takes-all variants. The US Presidential Election was decided by just 107,000 votes in three marginal states – Michigan, Pennsylvania and Wisconsin – and it elected a President who lost the popular vote by almost three million.

We can’t yet know the full impact of illicit influence or foreign intervention in that election. But we do know that the system offers the perfect environment for any such attempts.

The dominant parties in the UK have long defended First Past the Post, partly because the primacy of a handful of marginal seats makes it easier for them to game the system. But that same systemic weakness is precisely what now makes our democracy so vulnerable to unscrupulous, unaccountable, and often unknown forces.

The time for Proportional Representation in the UK is long overdue. These emerging threats add a new and pressing reason to scrap First Past the Post.