General election 2019: What is the secret behind tactical voting?

The result of the 2019 general election could determine whether Brexit happens or not.

Some parties have withdrawn candidates to help their rivals. Meanwhile, campaign groups are encouraging people to use tactical-voting websites, to help them decide which candidate to support.

So, what are election pacts and tactical voting - and how does each work?

Why do parties withdraw their own candidates?

Parties and candidates themselves sometimes make tactical decisions.

This generally means one party agreeing not to stand candidates in certain areas, so that another party with similar views on key issues has a stronger change of winning.

The aim is to stop the vote splitting, which could help rivals with very different views from taking the seat.

In Canterbury, Liberal Democrat candidate Tim Walker has stood down because he feared dividing the Remain vote. He said he was concerned standing would allow the Conservative candidate to take the seat from Labour.

And the Brexit Party has announced it will not contest the 317 seats won by the Conservatives in 2017. This is so as not to split the pro-Brexit vote in those areas. Instead, it says it will concentrate its efforts on taking seats from Labour, whom it suggested had "betrayed" its Leave-supporting voters.

The Green Party has stood down its candidate in Chingford and Woodford Green, to help Labour try to unseat former Tory leader Iain Duncan Smith.

Separately, anti-Brexit parties Plaid Cymru, the Liberal Democrats and the Greens have agreed an electoral pact in 11 of the 40 seats in Wales. The aim is to get as many Remain-supporting MPs elected as possible.

Back in August, the Lib Dems beat the Conservatives in the Brecon and Radnorshire by-election, after Plaid Cymru and the Greens agreed not to stand.

Labour has ruled out forming pacts with other parties.

What is tactical voting?

Put simply, tactical voting is when any of the UK's voters chooses to back a candidate they wouldn't normally support. This is done in the hope of stopping another candidate winning.

This could happen in a constituency where two parties are in a tight race and candidates from other parties trail far behind.

In these circumstances, a supporter of the candidate who was a distant third, might pick their favourite of the two who are in with a chance.

At this election, campaigners say voting tactically could help MPs who share voters' views on Brexit win more seats.

A number of tactical voting websites have been set up, but there is criticism over the nature of their advice.

Why would tactical voting and election pacts matter in the UK?

In a general election, the UK's voters are invited to choose an MP for their area - one of 650 constituencies.

The UK uses a first-past-the-post voting system, sometimes described as "winner takes all".

It means the local candidate who receives the most votes becomes an MP - and the party with the most winning candidates nationally normally forms a government.

Coming a good second does not help a party win any more power in Parliament.

Tactical voting is perfectly legal in the UK. Voters are free to choose any candidate on the ballot paper, no matter what the reason is.

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Will tactical voting increase at this election?

This election is going to be highly unpredictable, researchers at the British Election Study say.

That's because voters could be more likely to switch their support, largely because of the uncertainty over Brexit.

Last month, the Electoral Reform Society campaign group asked polling company BMG Research to find out how widespread tactical voting might be.

Of 1,500 voters questioned, 24% said they planned to vote tactically to keep out a candidate they dislike.

That compares with 66% who said they would vote for their first preference - regardless of how likely they were to win. The remaining 10% said they didn't know.

When the same question was asked before the 2017 general election, 20% of people said they planned to vote tactically.

Is there any evidence tactical voting works?

Tactical voting has been a very powerful tool in the past, says Stephen Fisher, professor of political sociology at the University of Oxford.

"It played a big role in delivering a landslide for Labour's Tony Blair in 1997 and it's been a staple of elections since then," he says.