FOR SOME weeks Boris Johnson, Britain’s prime minister, has been demanding an early general election. But he has been stymied because the 2011 Fixed-term Parliaments Act (FtPA) requires a two-thirds majority of MPs to agree to such an election—and the Labour opposition has refused to vote for one. Now Mr Johnson has got his way after parliament voted by 438 to 20 to hold an election on December 12th. Jeremy Corbyn, the Labour leader, had belatedly come round, now that his condition of avoiding any risk of falling out of the European Union without a deal had been met, thanks to the EU’s decision this week to extend the Brexit deadline from October 31st to January 31st 2020.

Mr Johnson wanted the December election because he is confident of winning. The Tories have a double-digit poll lead over Labour (see our poll tracker). As a newish party leader and prime minister, he believes he can campaign more effectively than his opponents. He has just won a new Brexit deal in Brussels, enabling him to appeal to voters under the slogan of “let’s get Brexit done”. Mr Corbyn’s novelty value has faded after four years as Labour leader and he is plumbing new depths as the most unpopular opposition leader in 45 years. Yet despite this the election remains a big gamble for the prime minister, for several reasons.

One is that the electorate has become more volatile. Party loyalty is much weaker than it was and switching parties between elections is more common. Many voters now identify more strongly with having been Remain or Leave supporters in the 2016 referendum than with either of the two main parties. An election in which four parties (the Conservatives, Labour, the Liberal Democrats and the Brexit Party) are running in double figures in the opinion polls, and in which tactical voting may be widespread, is inherently unpredictable, as many political analysts have conceded.

Second, there are several parts of the country where the Tories are likely to lose rather than gain seats. These include strong Remain areas such as London, parts of the south-east and university towns. The Lib Dems are hoping to make substantial gains in these places, and also in their traditional strongholds in the south-west. In Scotland the Scottish National Party (SNP) is expecting to win more seats, not least because the Conservatives have lost Ruth Davidson, a popular Scottish Tory leader. Mr Johnson hopes to more than make up for such losses by winning over Labour Leavers in the north and midlands, but this may be harder than he thinks. Although they may have backed Brexit in 2016, many such people have never voted Tory in their lives.

Indeed, Brexit is potentially a third awkward problem. Mr Johnson is trumpeting the new deal he has secured, but he is also seen to have abandoned his bid to push it through Parliament. That has meant missing his self-imposed deadline of delivering Brexit by October 31st, “do or die”. The election can expect to feature many replays of his declaration that he would rather be dead in a ditch than ask for an extension of the deadline, which he has now had to do. He may succeed in blaming recalcitrant MPs for failing to pass Brexit. But the Brexit Party under Nigel Farage is likely to run candidates against the Tories on a platform of not trusting Mr Johnson and preferring a no-deal Brexit instead.

Fourth, though Mr Johnson hopes to fight the election on the basis of his ability to get Brexit done, voters could choose to focus on other issues. Mr Johnson promises to spend more on the National Health Service and schools and to hire more police officers. Yet the Tories have been in office for nine years, and they are still blamed by many for a long period of austerity and public-spending cuts. If the fight turns into one over who can do most to boost public services, Labour (and the Lib Dems) have a natural advantage.

The betting is still that, with his poll lead and high popularity, Mr Johnson will win a majority in December. But there is much scope for the election to go wrong for him. When his predecessor, Theresa May, announced in April 2017 that she was calling an election in June, she had a far bigger poll lead than Mr Johnson has now. Yet when the results came in, she had lost her majority and faced a hung parliament instead.