How reliable are opinion polls when it comes to by-elections?

Two surveys suggest that the Conservatives will lose to Labour in both Uxbridge and Selby, writes John Rentoul

Keir Starmer’s Labour Party has the lead in a number of by-election polls (PA)

Several opinion polls have been carried out in constituencies where by-elections are under way or imminent, providing a snapshot of how the parties stand.

In Uxbridge and South Ruislip, where Boris Johnson stood down rather than risk being “recalled” by his constituents, Labour’s Danny Beales is eight percentage points ahead of the Conservative candidate, Steve Tuckwell.

In Selby and Ainsty in North Yorkshire, where Nigel Adams, Johnson’s ally, resigned after not getting a peerage, Labour’s Keir Mather is 12 points ahead of the Tories, who have yet to choose their candidate.

These polls were carried out by JL Partners, run by James Johnson, Theresa May’s pollster when she was prime minister, using a relatively new technique for conducting constituency polls online.

In the past, constituency polls would be carried out by telephone, but few people answer landline calls these days, and those who do tend to be old and unrepresentative. Pollsters responded to this change by including mobile numbers – using modelling programmes that can guess if a mobile user lives in a particular place – but nobody answers unsolicited calls on mobiles, either. People are more likely to respond to surveys by text message, but won’t usually answer as many questions as are needed to construct a demographically representative sample.

So JL Partners worked with Matt Singh of Number Cruncher, who pioneered a technique called “online river sampling” to produce accurate internet surveys of single constituencies. Singh was within one percentage point of the Lib Dems’ winning share of the vote in the Brecon and Radnorshire by-election in 2019, and again within one point of Labour’s winning margin in the Wakefield by-election last year.

Some pollsters have warned that accurate surveys are hard to carry out in single parliamentary seats.

Still, there are good reasons for thinking that Labour is well placed to win in both Uxbridge and Selby. We can ignore an earlier poll carried out in Uxbridge by Michael Ashcroft, the Tory peer and pollster, last month. Not only was that done using telephone interviews, with a sample heavily skewed towards older voters, but it was before Johnson announced his resignation last month. For what it is worth, it suggested that, if he were a candidate in a by-election, he would hold the seat.

Another difficulty in polling by-elections is that opinion can shift dramatically during the campaign, as voters realise that they have a chance to send a message to the government, or as they work out which candidate is best placed to deliver that message. But those shifts tend to work against the candidate of the governing party, so the two JL Partners polls suggest that the Tories’ chances of holding on to either seat are slim.

The same applies in the third by-election to be held on 20 July, in Somerton and Frome, in Somerset, where David Warburton, the former Conservative, stood down – though the findings against him have since been set aside and a new investigation ordered. Somerton and Frome is such an obvious target for the Lib Dems, who held the seat for 18 years before Warburton won it in 2015, that no one has even bothered to commission an opinion poll.

It looks similar to Tiverton and Honiton in Devon, which Richard Foord won for the Lib Dems with a huge swing last year – after the resignation of Neil Parish, the tractor porn MP.

Meanwhile there has also been a poll in Mid Bedfordshire, where Nadine Dorries, another Johnson supporter, said she was standing down with immediate effect but has yet to do so. She has put her resignation on hold while she tries to find out why she didn’t get a peerage in her hero’s resignation honours list. This poll, by Opinium, was commissioned by the Labour Party and published last weekend as part of the battle between Labour and the Liberal Democrats for the right to claim the title of “party best placed to defeat the Tory”. It showed Labour on 28 per cent, the Tories on 24 per cent and the Lib Dems on 15 per cent, with an independent on 19 per cent and Reform, the renamed Brexit Party, on 10 per cent.

It is still unclear whether Dorries will actually resign, but it is harder to predict which party will win her seat from the Tories should she eventually do so. Labour are fighting it hard, even though the Lib Dems could potentially win over more disaffected former Tory voters as the campaign progresses.

Meanwhile, there are two further likely by-elections. In Rutherglen and Hamilton West, Labour is confident of winning the seat from the Scottish National Party when Margaret Ferrier, the lockdown-breaking MP, is forced to step down. And Tamworth, the seat of Chris Pincher, the former Conservative deputy chief whip who was suspended from the Commons this week, is also likely to fall to Labour.

It is one of the safest Tory seats in the country, but with Labour an average of 22 points ahead in national opinion polls, no seat is safe.