Why the London result will decide the next General Election

Labour was miles behind in the polls when Theresa May called an election in 2017, but the party's leadership did not flinch. It is flinching now.

The party is in a mess in parliament and in the polls. It has lost the trust of Remainers in an election that is set to be about Brexit, rather than the leadership's preferred territory of austerity and inequality. Labour will be relying on a tactically voting electorate to save the party from itself.

In much of the country, this is at least plausible. But in London, it could fall apart.

The capital is shaping up to be England's most significant electoral battleground. That isn't because London has more marginal seats than other regions, but because its marginals are three-way battlegrounds. The combination of residual Tory voters, established Labour core votes and Lib Dem-friendly demographics mean that London's marginals could plausibly be won by any of the three main parties, whereas the rest of England is a series of head-to-head contests.

Whatever gains the Tories make in the North, Midlands and Wales must be balanced against the seats they are set to lose in Scotland and London. But neither Scotland nor London voted wholly for Remain—40 per cent of the capital's voters backed Leave. The electoral fortunes of Labour, the Tories and the Lib Dems all rest heavily on London. And right now, the Tories are doing better in London than people realise.

YouGov publish regional breakdowns in their regular voter intention polling. They come with health warnings - the sample sizes for each region are small and the margin of error consequently large, and YouGov is only one pollster at a time when different polling firms are showing markedly different Tory leads. But they do give a sense of broad trends and patterns around the country.

In the 2017 general election, the Tories got 33 per cent of the vote in London, compared to 55 per cent for Labour and just nine per cent for the Lib Dems. YouGov's first post-election poll, in July that year, had the Tories at 29 per cent in London and Labour on 57 per cent, as Labour opened up a national poll lead.

But taking the average of the five most recent YouGov polls, going back to late September, we see that Labour's support has collapsed in the capital to 29 per cent, behind the Tories on 30 per cent, with the Lib Dems surging to 28 per cent. A near-30 point lead in July 2017 has completely evaporated.

This is not unique to London. Labour is a distant third in the South outside London, far behind the Tories across the marginal-heavy Midlands and Wales, and only just ahead of the Tories in the North, where it has huge vote shares in core cities but much smaller majorities outside them.

But in most battleground seats outside the capital, there is only one clear option for opponents of the Tories and Brexit: either Labour or the Lib Dems. This makes tactical voting a straightforward exercise. If tactical voting happens, the Tories are in trouble—if it doesn't, the Tories will clean up.

In London, the demographics are different. Some of the Tories' Remain-voting seats have Labour in second after the last election, but have demographics that are unfavourable to Jeremy Corbyn. These are middle class, liberal, professional voters, ardently anti-Brexit, who are not keen on Corbyn and are very keen on the Lib Dems' unapologetically pro-Remain line. The Barnet seats contain substantial Jewish populations who are alienated from Labour. Tactical voters in these seats are liable to be torn between Labour and the Lib Dems, potentially allowing the Tories to hold on.

Then there are some of Labour's gains from 2017. Labour has reliable core votes in these seats, but again, there are constituents who will be making eyes at the Lib Dems, potentially allowing the Tories back in.

The other big unknown is the South West. Here the issue is not tactical voting, given that many of these seats are straightforward Tory-Lib Dem battlegrounds in traditional yellow heartlands. But these are ageing, Leave-voting seats. If the Lib Dems' revival is uniform across the country, the Tories could unexpectedly lose seats across Cornwall, Devon and Somerset. If it's concentrated in Remainy London and the Home Counties, the Lib Dems could be extinguished in what was once their strongest area.

This is not conclusive. The polling might be wrong, for a start, or an election campaign could see a clear non-Tory option emerge. The Tory vote share might be concentrating in its pro-Leave Outer London strongholds while bleeding in the battlegrounds.

But given their predicted gains in Leave heartlands, if the Conservatives hold on in the capital, they are very likely to hold on in the country at large.