Gina Miller, the pro-EU campaigner, has launched the “biggest tactical voting drive ever,” raising nearly £300,000 to support parliamentary candidates from different parties who want to stop “Extreme Brexit”.

But could tactical voting really make a difference to the election on 8 June? Is there any chance we could end up with a “progressive alliance” of parties opposed to “hard Brexit”, as the Green Party have advocated?

Who would be included?

“There are two versions of a progressive alliance,” polling expert Professor John Curtice told us. “They’re closely aligned, but not quite the same.

“Version One is saying that any Labour, Liberal Democrat or Green winner is preferable to a Conservative winner. Version Two is that any pro-soft Brexit or Remain is preferable to any hard Brexit MP.

“The difference arises in cases like (pro-EU Tory) Ken Clarke and (Labour Leaver) Kate Hoey.”

In other words, a tactical voting drive that was purely about Brexit could in theory see Labour supporters voting Conservative, and vice versa.

Gina Miller’s campaign team didn’t answer FactCheck’s questions about whether they would support pro-EU Conservatives, saying details would be clarified soon.

Where does Labour sit in this tactical voting drive? The campaign page claims they will support “candidates who pledge to support a full and free vote on the Brexit deal”.

But, as political scientist Dr Chris Prosser points out: “Jeremy Corbyn has ruled out a second referendum, so the only way that Britain wouldn’t leave the EU is if there’s enough Lib Dems to form a government with the SNP – which is incredibly unlikely!”

The other problem is that voting priorities in Scotland and Northern Ireland are likely to be very different to those in England and Wales. In Northern Ireland, the DUP and the UUP are discussing an election pact, despite being on different sides of the Brexit debate.

Meanwhile, the Scottish Conservative leader, Ruth Davidson, is already calling for tactical voting to prevent a second Scottish independence referendum, saying the Conservatives are the only alternative to the SNP. So pro-EU tactical voting may not be nearly as effective, because other issues are on the table that could act against it.

How many extra seats would be needed?

Labour are currently miles off a majority, with just 229 seats compared to the Conservatives’ 330. But if you add in the SNP (54), Lib Dems (9), Plaid Cymru (3) and the Greens (1), this figure rises to 296. Plus, Labour will be hoping to keep its seat for Manchester Gorton, which is currently empty following the death of Sir Gerald Kaufman.

In theory, if these parties clubbed together they would only need 25 extra seats to get an effective majority (discounting the Speaker, the Deputy Speakers and Sinn Fein MPs, who abstain from parliament). Or, for a proper workable majority disregarding Sinn Fein’s abstention, 29 new seats.

It’s perfectly possible that tactical voting could help progressive parties make some gains in certain seats. After all, there are several extremely marginal constituencies, like Gower in Wales, where the Conservatives beat Labour by just 27 votes in 2015.

According to data from Electoral Calculus, if Labour, the Lib Dems, the SNP and Green voters had all coordinated perfectly in the 2015 election, they could have achieved a combined majority. But, of course, not only is that level of coordination impossible, there would also have been vast numbers of voters who wouldn’t want to vote tactically. And remember, those figures are based on 2015 levels of Labour support, before the party plummeted in the polls.

And even if a progressive alliance did manage to scrape a majority, the Conservatives would almost certainly still be the largest single party, meaning they could try forming a minority government or a coalition.

According to a YouGov poll last year, 43 per cent of people likely to vote in England said they supported a “progressive” party. But even if people are prepared to vote tactically, or for an alliance, they might be in the wrong constituencies to make a difference.

YouGov said: “While there is a high level of transferability within the ‘progressive’ pool, ultimately the pact would have a hard time securing electoral success. This is because the most enthusiastic supporters of a progressive alliance would add votes where they were least needed to elect left-of-centre MPs.”

Haven’t people been voting tactically for years?

Yes. According to Professor Curtice, tactical voting started occurring at an unprecedented level in the 1990s and already has an impact on elections. So it’s possible that many people who are prepared to vote tactically are already doing so.

“Getting more of it looks more difficult, unless Brexit proves to be such a crucial issue,” Curtice said.

Would there be a knock-on effect?

“If you make alliances, you change the incentives of how people vote,” Dr Prosser told us.

“And you might change the incentives of the people on the other side of politics vote. You can’t just change one piece of the puzzle when it comes to voting; everything is moving in tandem.

“There’s plenty of people who see themselves as progressive, but don’t like Labour, or the Lib Dems or the Greens, or they don’t want Scottish independence. If you say to them: ‘We’re going to come together and make Jeremy Corbyn Prime Minister’, I think there’s a lot of people who wouldn’t be OK with that.”

Would a progressive alliance work together?

Probably not.

Even if they had enough seats, it’s hard to see how these parties could ever form a coalition together – though it’s not impossible. Labour and the Lib Dems have already rejected calls for an election pact with other parties.

And although politicians sometimes go back on their word, at the moment it seems unlikely that Labour would ever hook up with the SNP. The parties have been highly critical of each other, with Corbyn saying he does not see them as “reliable allies”.

Professor Curtice explained: “South of the border, people occasionally say: ‘Wouldn’t it be nice if Labour and the SNP got together?’. The Labour party has just been crucified by the SNP. What the hell makes you think they’re going to get together? The Labour party neither wish to touch the Tories with a barge poll, or the SNP with a barge poll.

“You can try persuading Labour voters in Scotland to vote SNP, but I kind of just wish you the best of luck.”

Prosser added: “Unless something really drastic happens, like Scottish Labour being dissolved, I think it’s very unlikely. Labour is actually competing against the SNP in Scotland, so it’s very hard to see how that would work in terms of electoral competition.”

So is tactical voting worth it?

“On the face of it, it has an obvious appeal,” said Dr Prosser. “But it’s very hard to see how it’s likely to change the final outcome of the election, in terms of who wins and who loses.

“Tactical voting does make a difference in individual seats and, in the past, the Liberal Democrats in particular have strongly relied on people voting tactically. One of the problems they faced in 2015 was the evaporation of this tactical support.

“So it probably will make a difference in some individual seats, but with the Conservatives as far ahead as they are, it’s very unlikely to change the final outcome.”