What is tactical voting and could it decide the outcome of the election?

Since Theresa May announced her intention to call a snap election, a number of schemes encouraging supporters of opposition parties to vote tactically have emerged.

But what exactly is tactical voting? And could it affect the outcome on June 8?

What is tactical voting?

This is when constituents vote for a candidate who is not their first choice in order to prevent another party or candidate from winning in the area.

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This is usually because their preferred candidate’s chances of winning are slim to none, so the voter chooses to give their second-best option a greater chance of success.

When does it tend to happen?

It usually happens in marginal seats – where the constituency is only held by a small majority.

An example would be Sir Vince Cable’s old constituency of Twickenham – the Conservatives’ Tania Mathias won a small majority at the last election, closely followed by Liberal Democrat Sir Vince. With the former business secretary announcing he will stand again, it makes the seat a key target for the party and for tactical voters.

The hope among tactical voters would be that if enough people back the party in second place – in this case the Lib Dems – it would be enough to swing it in their favour and overthrow the current MP.

What is this document that everyone’s sharing?

On the day the snap general election was announced, a spreadsheet began circulating online called “How to vote to stop the Tories.”It highlights all the constituencies where tactical voting would apply, indicating whether willing participants should vote for Labour, the Lib Dems or someone else.It has been widely shared on Facebook and Twitter.What else is going on?

Gina Miller, whose legal action forced Theresa May to hold a parliamentary vote on her plans for Brexit, has launched “the country’s biggest tactical voting drive”.Miller said her crowdfunding campaign was aimed at preventing an “extreme Brexit” by securing the election of as many MPs as possible who were committed to a “real” final vote by Parliament on the terms of Britain’s withdrawal from the European Union.At the time of publishing, the page had passed the halfway mark with more than £6,300 raised. The money will be used to “back candidates who pledge to support a full and free vote on the Brexit deal”.

Caroline Lucas has also called for talks with Labour leader Jeremy Corbyn and Liberal Democrat leader Tim Farron to form a so-called “progressive alliance” against the Conservatives ahead of the election.While the Green Party will still have candidates standing across the country, co-leader Lucas has called for a co-operative approach “in a handful of places”.How much could tactical voting influence the upcoming election?

Stephen Fisher, professor of political sociology at the University of Oxford, believes the chances of success for any anti-Tory tactical voting scheme are fairly slim.He said: “If polls are roughly right and nothing much changes, one trouble for Labour will be that there simply are not very many Liberal Democrat supporters to appeal to.“So the potential for Labour seats to be saved by Liberal Democrat, and maybe Green, tactical voters is rather limited. In addition there is the potential for the Conservatives to be helped by tactical voting from Ukip supporters, making the job for Labour harder.“By contrast, in the seats that the Liberal Democrats lost to the Conservatives in 2015, and so gave David Cameron his majority, there are many Labour supporters who might well be willing to vote tactically for the Liberal Democrats.“Liberal Democrat success in their target seats off the back of Labour tactical support might help to limit the size of Theresa May’s majority, but there are not enough Conservative-Lib Dem seats for this to offset the likely number of Conservative gains from Labour.”