In recent years, elections and referendum results have made a mockery of political scientists’ predictions.

There are a myriad reasons for that – from the algorithms that drive advertising on social media to people not bothering to vote or changing their minds at the last minute.

One of the biggest headaches for pollsters as the UK gears up for a December 12 general election is how many people will vote tactically – and where.

Current polls suggest Boris Johnson holds a comfortable, if narrowing, national lead over Jeremy Corbyn with two and half weeks to go until December 12.

But targeted tactical voting in key individual seats across the country could deliver some big upsets – as it has done in the past. 

What is tactical voting? 

Simply put, tactical voting is when people abandon their traditional party loyalty and instead vote for the candidate seen as most likely to beat one they dislike.

It has been used to great effect in the past. Anti-Tory tactical voting by Lib Dem and Labour voters in 1997 arguably cost the Conservatives as many as 80 seats – including the famous defeat of Michael Portillo in Enfield Southgate.

Who does it benefit?

In this election, tactical voting is mostly being seen through the prism of Brexit.

Pro-Remain campaigners want to deny Boris Johnson a majority and prevent him from passing his Brexit deal through the Commons before Christmas. A series of nationwide campaigns targeting key seats are already underway.

Jo Swinson, the Lib Dem leader, has argued her party stands the best chance of winning Tory-held seats and blocking Johnson from gaining that majority. The party is targeting a number of Conservative constituencies, including Dominic Raab’s Esher and Walton, and asking the anti-Brexit vote to unite behind its candidate.

Nigel Farage, meanwhile, has pleaded with pro-Brexit Conservative supporters to vote tactically for Brexit Party candidates in seats where his party could stand a better chance than the Tory candidate of ousting the Labour incumbent.

Who does it hurt?

At this election, most of the noise around tactical voting appears mostly to be targeted at the Conservatives. The Sunday Times has reported Boris Johnson, Iain Duncan Smith, Steve Baker, Zac Goldsmith, Philip Davies Sir John Redwood are all potentially at risk.

The calculation was based on an analysis of putting private poll findings and 270,000 voter interviews conducted by YouGov through a Datapraxis computer model, known in the polling industry as MRP, which analyses the specific demographics of each seat.

How do you vote tactically?

This election there are several tools, from pro-Remain groups, designed to direct voters how to vote tactically if they want to in their seat.

Best for Britain, the pro-EU campaign group, has calculated the Tories will fall short if just 30% of pro-Remain voters act tactically to unite behind a pro-Remain candidate. It has produced a website designed to help voters decide who to back. The People’s Vote campaign also has a calculator, as does leading Remain campaigner Gina Miller.

Arron Banks, the founder of Leave.EU, has also promised to launch a tactical voting app that would advise people to vote Tory to ensure Johnson wins a majority.

The websites are not without their critics, however. Labour has said the Best for Britain model unfairly encourages people to vote Lib Dem, even if the party came a distant third in 2017.

How much tactical voting will there be? 

It is hard to know. A recent Ipsos-MORI poll revealed most voters (70%) said they will vote for the party that best represents their views. This is fewer than we saw in the run-up to the 2017 and 2015 elections (75% in May 2017 and 85% in May 2015). But just 14% say they’ll vote for a party specifically to try to keep another party out.

A BMG survey for The Independent suggested 31% of voters were prepared to vote for a candidate best positioned to beat one from a party they dislike. The survey showed 31 per cent of Remainers and 30 per cent of Brexiteers were prepared to vote tactically.

Professor Tim Bale, the deputy director of the UK in a Changing Europe think-tank, said the potential of tactical voting could be exaggerated. 

“Tactical voting has the potential to swing elections in individual seats, obviously, but I’d guess it won’t be sufficient to prevent the Tories winning a majority if they’ve got a decent double digit (or near-double digit) lead over Labour,” he said.

“Depending on which poll you look at, you can find anything from fewer than one in 10 to not far short of a quarter of voters saying they’re tempted to vote tactically.

“And the web makes it much easier nowadays to identify who you should be supporting as the most likely candidate to defeat whichever party you really don’t like.

“That said, we can only guess at how many people will take the trouble to figure that out, whether they’ll come up with the right answer if they do, and whether ultimately they’ll go with their heart or their head on polling day.”

Pollster Lewis Baston agreed the question of how much tactical voting would take place was almost “unanswerable”.

“The usual level is around 10%,” he said. “Expect it a bit higher this time given we have such a strong political Leave/Remain identity now.

“But to some extent because party loyalty is so weak it’s hard to tell tactical from ‘sincere’ voting – someone who felt Brexit Party a couple of months ago and now intends to vote Conservative. Are they tactical voters or converts?”