Beware projections of parliamentary seats from national polls. This weekend’s survey of three marginals suggests that voting movements are likely to vary wildly – even in neighbouring seats which look similar.

Conservatives open up 19-point lead with 47% share of the vote Read more

In the Cities of London and Westminster, Chuka Umunna could become the first non-Conservative MP for the area since 1874. The former Labour MP, now fighting the seat as a Liberal Democrat, is currently six points behind Nickie Aiken, the Conservative candidate. But Deltapoll’s figures show that tactical voting by Labour party supporters could give Umunna victory. In contrast, Hendon, a highly marginal seat that Labour held from 1997 until 2010, could see the Conservative majority jump sharply, with the Lib Dems marooned in third place.

Over in Chelsea and Fulham, the poll dents Lib Dem hopes that tactical voting could help Nicola Horlick, the financier and one of their high-profile candidates, overturn a Conservative majority of more than 8,000. She and Labour’s Matt Uberoi both lag more than 20 points behind Greg Hands, the former minister defending the seat for the Conservatives.

Hands is benefitting from being more pro-EU than most Tories. Chelsea and Fulham opposed Brexit by more than two-to-one in the referendum. Remainers who voted Conservative in 2017, a sizeable proportion of the local electorate, have stayed more loyal to Hands than Remain-minded Tories in other seats.

Taking last weekend’s poll results together with this weekend’s, we have figures for six London seats that all voted Remain three years ago. (Next weekend we shall have results from other parts of the country.) Two main lessons can be drawn from those six seats.

In four seats the Lib Dems have made huge gains, and are now the main challengers to local Tories. But they have advanced far less in the other two. By the same token, Labour support is down in all six seats, but by amounts ranging from nine to 25 percentage points.

The most striking contrast is between Hendon and neighbouring Finchley and Golders Green, whose data we reported last weekend. Both are Remain-voting suburbs that the Tories held by fewer than 2,000 votes two years ago. They are the two constituencies with Britain’s highest proportion of Jewish voters. The Lib Dem vote last time was below the party’s meagre national average in both seats.

Not surprisingly, the poll models used to project local constituencies from detailed national surveys – they are known in the trade as “MRP” projections – produce very similar predictions for the two seats. But our actual polls yield very different figures. With tactical voting, Luciana Berger has a real chance of winning Finchley for the Lib Dems, but Clareine Enderby, her party colleague next door, is trailing a distant third. Local factors such as the impact of candidates and the effectiveness of local campaigning make a difference – and that difference might be greater this time than in past elections.

The latest surveys also underline what we found last weekend: that Labour supporters are far more willing to switch tactically to the Lib Dems than vice versa. The subsamples from each seat are small; but when we combine the figures from all six, the pattern is clear.

Asked how they would vote if Labour had no chance of winning locally, 57% of Labour supporters say they would switch to the Lib Dems, and only 9% to the Tories. But if Lib Dem supporters thought they had no chance of victory, just 37% would switch to Labour, while 16% would vote Tory.

We shall see next weekend whether this pattern is repeated outside London. If it is, then the Conservatives have more reason to worry where they are facing a strong Lib Dem challenge than where their main local opponent is Labour.

Peter Kellner is a polling expert and former director of YouGov