Even before day one of Theresa May’s prime ministership, she was categorical about the undesirability of an early election. As Mrs May put it in Whitehall on 30 June 2016 at the launch of her Conservative leadership bid: “There should be no general election until 2020.” This was unambiguous and, until today, it was the mantra to which she has stuck ever since. Her spokesman said it most recently last month. Asked if there might be an early election, Downing Street said it was “not going to happen”.

Yet now it is going to happen after all, and it is happening solely because Mrs May sees Conservative partisan advantage in making it happen. Today, Mrs May stood in Downing Street and announced with a completely straight face that the government intended to call an election on 8 June. Parliament will vote on the matter tomorrow. Unless more than a third of MPs vote against Mrs May’s motion, Britain is heading to the polls in seven weeks’ time. Mrs May may have presented the decision as the government’s, but it is very clear that it was hers alone.

So let us be very clear. Britain does not need, and its people are not demanding, this general election. There is no crisis in the government. Mrs May is not losing votes in the Commons. The House of Lords is not defying her. No legislation is at risk. There is no war and no economic crisis. Brexit is two years away. The press are not clamouring for an early election. The government has not run out of ideas. The opposition is not ready. Mrs May is enough of a Tory to know that British prime ministers who take over in midterm have no constitutional “need” of a personal mandate, especially when they are in as commanding a position as she currently enjoys. Yet now a supposedly five-year parliament will have lasted for just two, solely because Mrs May thinks this is a good time to crush Jeremy Corbyn’s Labour party.

As U-turns go, it is an absolute screecher. The smell of rubber on the Downing Street black top is acrid and foul. Judgments about Mrs May will never be quite the same, and deservedly so. She has built her authority by being, and by appearing to be, a leader who plays straight, gets on with the job and takes politics seriously. Like Alice in Wonderland, she said what she meant and meant what she said. Until 11am today that was more or less believable – and to her credit. Politics in general was the stronger for it.

But now there is a new dimension to Mrs May. She is now a party political leader whose words can’t be trusted at face value as much, and for whom politics is, after all, a game. The Tory party may win, if opinion polls can be believed, because Mrs May is trusted far more than Mr Corbyn. But the loss to wider politics ought to be severe. The damage inflicted by the hypocrisy of the apparently sincere is more serious than the damage inflicted by the transparently untrustworthy.

The likelihood that Mrs May will get her way in the Commons does not make it the right outcome. Under the Fixed-term Parliaments Act, if more than a third of MPs vote against tomorrow’s motion, there will be no election on 8 June. That minority veto exists for a serious and good reason: to prevent a dominant government party from steamrolling its opponents into an early poll. Labour, on its own, has enough MPs to block the move, even with the SNP and the Liberal Democrats voting with the government. Yet it is not going to happen because Labour is intimidated and fatalistic. The motion itself, and the likely Commons endorsement of it, are evidence of the weakness of our party politics, not their robustness.

Some of Mrs May’s reasons for calling the election are particularly unacceptable. To say, as she did, that a poll is needed because “division at Westminster” is causing “damaging uncertainty and instability” sails troublingly close to being a Thames Valley version of the sort of thing that President Erdoğan might say in Turkey. Division in parliament is necessary and inherent, above all on something as momentous as the Brexit terms. Brexit reflects life-influencing divisions in the country. Mrs May’s decision and language illustrate the damage that referendums do to parliamentary democracy.

Play Video 6:52 Prime minister Theresa May calls general election for 8 June – full video statement

The election is also an invitation to voters to buy Mrs May’s Brexit terms sight unseen. She said today that she wants support “for the decisions I must take”. But we do not know what those decisions will be. They depend on negotiations that have barely begun with some EU partners who face elections of their own, as well as on events. All this will involve give and take. Mrs May is seeking a mandate to do something of which not even she knows the main planks, the details and the trade-offs. She wants to get parliament off her back in making the Brexit terms. This election must ensure that this does not happen.

Many things may change over the coming weeks. At this early stage the danger is that the 2017 election may be less a contest about who should govern and more a contest about how much power the voters are willing to entrust to Mrs May. The Tory manifesto will have to be watched like a hawk; it will be an unusually crucial document. This is a premature election which the country does not need, the people do not want and Mrs May does not require in order to do her job effectively. Above all else, this election must not write her a blank cheque over Europe.