They blame the Rev Hubert Brasier. It was in her father’s vicarage that the young Theresa was taught self-restraint. The lesson was learned far too well. So think the Conservative MPs who reckon their leader is abstinent to a fault whenever she dismisses the idea of an early general election. Some admire this as evidence of Mrs May’s steely self-possession; at least as many view her disavowals as a mad and maddening refusal to cash in when the going looks so good for their party. If her reluctance to go to the country bewilders many a Tory, it utterly baffles the parliamentary Labour party. “It’s weird. I simply don’t understand it,” says one of their number. Labour MPs are amazed that Mrs May is not preparing to cast them into the jaws of the electoral slaughter that they are certain would engulf them if they were obliged to try to sell the country the concept of prime minister Corbyn.

Thinking about an early election is always bubbling in the hive mind of Westminster and the temperature has been raised in the wake of the recent byelections. By taking Copeland from Labour, Mrs May became the first prime minister to bag a seat from the opposition in 35 years. The last time that happened was in 1982 and Margaret Thatcher followed up with a general election the following year that rewarded the Conservatives with a landslide. Those looking further back for historical guidance can cite the Hull North byelection of 1966. A comfortable win for his party in the marginal seat persuaded Harold Wilson that a snap election would pay a handsome dividend. The Labour prime minister took his opportunity and by doing so converted a tiny Commons majority into a plump one. More than a few Tory MPs yearn for Mrs May to locate her inner Wilson.

She has motives aplenty. It has become fashionable to describe this prime minister as “dominant”. I also read that she is “impregnable” and as “unassailable” as Mrs Thatcher and Tony Blair at their zenith. Yet that impression of unchallenged rule is more a product of the lack of competition than respect earned by anything that she has done. She stands in the flattering light cast by the enfeebled condition of her opponents.

Blair and Thatcher had moats around their prime ministerial authority that were provided by landslide majorities obtained in their own names. Mrs May has a weak majority in the Commons and a mandate borrowed from her predecessor. That looks like feeble insulation against the challenging weather on the horizon. There will be a taste of her underlying vulnerability to backbench revolt and external discontent in this week’s budget. Against his own instincts to be the flinty fiscal disciplinarian and hoard money for rainier days, Philip Hammond will be forced to spray around some extra cash – here a bit on social care, there a bit on business rates – to try to douse rebellious fires that have been lit on the Tory backbenches.

Mrs May lacks her own mandate. Until she fights and wins an election in her own right, she can never claim a personal stamp of approval at the ballot box from the British people. That doesn’t seem to bother much of the public at the moment; her approval ratings are highly positive. The lack of a direct contract with the electorate will start to matter – and matter a lot – when her crown loses its lustre.

The opinion polls put the Tories in the 40s and with a giant lead over Labour. That suggests she would convert a precarious position in parliament into a powerful majority if she sought a decision from the country about who should be its leader.

She is currently blessed that so many of her opponents are in utter disarray. The party that is supposed to be her principal opposition is in no fit state to fight anything but itself. Labour is in a competition to see which it can lose faster: members, voters or self-respect. The Corbynista revolution is beginning to consume itself as one-time cheerleaders at Westminster and in the media either fall silent or turn on the man they once recommended to Labour’s members. Yet though there may be a growing consensus across the spectrum of Labour opinion that Mr Corbyn cannot last, this is not the same as any agreement about how he might be induced to spend more time on his allotment or who ought to replace him.

Talk to Labour MPs with majorities of 5,000 or less and you will find few who sound sure that they could survive a general election. Talk to Labour MPs with majorities in five figures and not all of them sound confident about saving their seats.

Then there’s the spectacle, one increasingly pleasing to Tory eyes, which is presented by Ukip. The plum and custard brigade used to frighten the life out of Conservatives. It so scared David Cameron that Britain is on its way out of the European Union. Like the Corbynistas, Ukip is now eating itself. The current leader, Paul Nuttall, has provoked the ire of the former one, Nigel Farage, for failing to win the Stoke byelection. A source of even greater Farage fury is the wilful refusal to acknowledge his greatness by granting him the title “Sir Nigel”. He demands the expulsion of Douglas Carswell, the party’s only MP, for the appalling sin of being unwilling to lobby to secure him a knighthood.

There are Conservative MPs in areas where the Remain vote was strong who are a bit troubled by the Lib Dems, but even for some of those Tories an early election would commend itself as a way to pre-empt a revival of the yellow peril before it becomes more menacing in their seats.

So Mrs May has plenty of motives for calling an early election. She will also soon have opportunity. She has told us that she will trigger Article 50 this month. Once she has sent the formal notification that Britain intends to withdraw from the EU, she will be able to appear in front of microphones with the argument that she is delivering on the instruction from the referendum. She could further say that she has laid out her broad strategy for the negotiations and now seeks the endorsement of the ballot box to execute her version of Brexit. This – or so think many of her MPs – furnishes her with a perfectly adequate justification for going to the country.

That’s motive and opportunity covered. Means are trickier. Before 2010, a prime minister could get an election simply by heading down to Buckingham Palace to ask for one. Now there is the obstacle of the Fixed-term Parliaments Act to be navigated. It certainly makes life more complicated for the prime minister. Still, where there was a will, many of her MPs think, Mrs May could find a way. The simplest mechanism would be to ask parliament to approve an election and dare the opposition parties to say no and make themselves look scared of the people. Quite a lot of Labour MPs would even relish it in a kind of way to get the misery of the Corbyn era behind them.

So if it is screamingly obvious to so many others, among them members of her cabinet, that she ought to go for an early election, what stops Mrs May? That’s not complicated. She declared against the idea when she first became prime minister and she is not renowned as someone who changes her mind lightly or easily admits that she has done so. An early election doesn’t fit with her self-image and public projection as someone who is above playing opportunistic games. The thought of an election will make a risk-averse politician nervous. Even the coolest customers who have resided at Number 10 have been anxious about calling elections – even when they had nothing to be worried about. Added to which is Mrs May’s belief that there is no pressing need to hurry because Labour is in such a distressed condition that it will not present any serious competition however long she waits.

Yet this is much more of a gamble than she appears to know. She is taking not just one punt on the future, she is staking an accumulator of wagers. She is betting big that Brexit will be negotiated successfully and to timetable and that the deal she comes back with, if she returns with a deal at all, won’t divide the country between the disappointed, the furious and the betrayed. Related to that is the wager that she is taking on the economy. She is betting that living standards will continue to do tolerably well all the way to 2020 and that Labour will carry on murdering itself.

She only has to be wrong about one of those assumptions – the outcome of Brexit, the health of the economy and the state of the political competition – and things could look very different. The impregnable May will become the embattled May. Tories will contemplate her failure to call an early election when she had a golden opportunity and find it hard to forgive her. Those who are at her feet today could be at her throat tomorrow.

By ruling out an election, Mrs May is taking a rather enormous bet that all versions of tomorrow will look pretty much the same as today. The vicar’s daughter is actually taking quite the gamble.