You might think that Tom Watson has a pretty safe seat. Labour’s Deputy Leader and MP for West Bromwich East enjoys a majority over 9,400, which he increased from 6,700 at the 2015 election, only two years ago. In fact, Labour have won every single election in the seat since it was created in 1974. They’ve had a recent boost in the polls, and could even beat their share of the vote in 2015. What could possibly go wrong?

Tom Watson is in big trouble. The sky is falling. Blue meteors are hurtling toward his head, spewing smoke and fire across the skies of West Bromwich. Swarms of purple locusts muttering about foreigners have turned the sky black and plunged the town into darkness. A town full of people who could barely even tell you what a Tory looked like ten years ago could be on the verge of electing one next week.

On the face of it, this seems absurd. How can there be such a big disparity between vote share and results? Tell that to Hillary Clinton, who won the popular vote in 2016. Sadly, that didn’t translate into the seats she needed to win the election; and so here we all are, glued to the news, watching Trump’s presidency circle the drain like a big angry satsuma slowly spiralling into a black hole.

National vote share can be misleading. It was true in the US Election last November, and a growing pile of data suggests that it could be true in the UK next month. To find out more, I teamed up with Martin Baxter from Electoral Calculus to investigate.

In the United States, individual states are polled regularly during campaigns. In Britain, we simply don’t have that kind of rich data. The best we have are YouGov’s national polls, which have been helpfully broken down by region. These numbers are based on smaller sub-samples and likely to be more error prone, which makes exact predictions about seat counts riskier. That doesn’t make them useless however; we can see a pretty clear trend, and it suggests more bad news for Labour.

The Conservatives won the popular vote in 2015 by 36.1% to 30.4%, but the result was different in each region. in the North-East Labour led by 46.9% to 25.3%, while in the West Midlands the Tories took 41.8% to Labour’s 32.9%. For a given national vote share, you can predict a range of different outcomes in different parts of the country.

Regional deviation from expected results based on national polling.

YouGov’s results show that the regional variations aren’t quite what we’d expect from the headline poll numbers. In the North-East for example, the Tories are 7 points higher than you’d predict, while Labour are 3 points lower – a potential swing of 10 points against the national polls. The same story plays out to varying degrees across the north, Yorkshire, the Midlands and Wales – Labour seen to be doing worse than headline numbers suggest, while the Conservatives are doing better. The only regions to buck the trend are the South-West and South-East.

Then there’s the Brexit effect. A few weeks ago, we looked at how Remain and Leave voters were behaving, and noticed that the Lib Dems could be in big trouble. While Leave voters have deserted UKIP en masse to bolster the Tories, there’s been little evidence of tactical voting among Remain voters.

At the time, we were sticking our necks out against a firm consensus among pundits that the Lib Dems would do well. Poor local election results in May seem to have put paid to that view, but UKIP’s announcement that they only plan to stand in 300-odd seats – leaving 100s of thousands of voters to swing back to the Tories – has made a bad situation even worse for the Lib Dems. It could also heap misery on Labour.

Which brings me back to Tom Watson. Watson leads by 9,400 votes over the Conservatives in West Bromwich East. That sounds great until you realise that there were also 8,000 UKIP voters in 2015, and UKIP won’t be putting forward a candidate. Thousands of those voters are likely to turn up and put a cross next to the name Emma Crane, the Tory candidate.

Regional trends in polling make the problem even worse. West Bromwich East is in the West Midlands. YouGov’s polling suggest that the Tories are doing about two percentage points higher than expected in that region, while Labour are doing about 3 percentage points worse. Take that five-point swing and add it to those thousands of UKIP voters, and a 9,000+ majority suddenly looks razor thin. I’m not sure I’d bet against Watson, but if he survives it will be by a narrow margin.

Tim Farron, the Lib Dem leader, could be vulnerable to a similar combination of effects. Like Watson, he has an apparently-safe majority of around 9,000 over the Conservatives. Like Watson, he has an orphaned pool of UKIP voters to contend with; but they only number about 3,000. Enough to dent his lead, but manageable.

Farron’s big problem is the regional trend. His constituency of Wesmorland and Lonsdale is in the North-West. National polling already has the Tories about ten percentage points higher than in 2015, but in the North-West they’re outperforming national polls by another 3 points, while the Liberal Democrats lag slightly. Like Watson, I wouldn’t bet against Farron keeping his seat. What’s clear though is that many of these safe seats are far closer than people might assume.

Recent polls show the Labour party recovering slightly to an average of about 31%. That’s roughly the vote share they achieved in 2015, which might lead you to assume they’ll win a similar number of seats. In fact, after adding regional trends to the model we used to investigate the Brexit impact a few weeks ago, and feeding in the most recent poll from each of the 8 main polling companies, Labour still look set to lose dozens of seats. The 232 seats they have now could be reduced to between 164 and 173 seats, with the Tories winning 406 to 417.

That may be too optimistic. For all the excitement, Labour’s polling is still far worse than it was at the last election, when its vote share was over-estimated in predictions. Two years ago, 33-34% in the polls translated into a 31% vote share. A 31% vote share today could easily become a 28 or 29% vote share on election day, in which case a seat count in the 140s is far from out of the question. Whatever the precise numbers turn out to be, a lot of well-known MPs face a very uncomfortable night on 8 June.