I have been trying – so far, I confess, without success – to discover who minted the phrase “political honeymoon”. It is a strange expression: marriage is rarely an appropriate metaphor for a country’s relationship with its leader. It describes an odd quirk of electorates. The public tell pollsters that they are most enthusiastically in favour of a leader in his or her opening period in office, precisely the time when voters know least about the person who has just taken charge.

Theresa May is benefiting from this irrational phenomenon and enjoying much higher approval ratings than those of David Cameron during his final chapter at Number 10. The British are often held to be a cynical bunch, but this country usually awards some sort of honeymoon to a new prime minister. John Major had one when he succeeded Margaret Thatcher and pulled off the clever trick of making the country think that they’d got a new government as well as a new leader. Tony Blair had a long and even intense one after he won his first election in 1997. Hard though it may be to recall now, Labour didn’t lose a seat at a byelection in his first six years at Number 10. At one point, his pollsters were telling him that Mr Blair had an approval rating of above 90%, the sort of numbers you would usually only expect for the leader of North Korea.

Gordon Brown had such a hot romance with the electorate when he first moved into Number 10 that he was tempted to call a snap general election. Then he got cold feet and that was the end of his honeymoon. David Cameron had a relatively long period of grace with the electorate after 2010, partly because a lot of the flak for unpopular decisions in the early years of the coalition was deflected on to the Lib Dems.

The honeymoon being enjoyed by Mrs May has some features specific to her personality and circumstances. Since she arrived at Number 10, she has combined the novelty of change with the reassurance of the familiar. She was a well-known, if not that well-understood, figure after a long stretch as home secretary. She looks the part as prime minister. After all the turbulence around the Brexit referendum and its aftermath, she has come across as a stabilising presence. At the same time, she is also something different, the first female prime minister in more than a quarter of a century and only the second woman at Number 10 in our history. The media, as is usually the case with a new prime minister, are being reasonably well-disposed towards her. In her honeymoon period, she can get a positive headline out of the admission that she doesn’t like the shape of her nose. The Tory press like her, at least for now. They never really warmed to her predecessor; the rightwing tabloids hated Mr Cameron for the Leveson inquiry and they then went to war against him over Europe. The leftish media are still calibrating their gunsights or too preoccupied with the self-immolation of the Labour party to be concentrating on Mrs May.

Defining yourself against your predecessor, even when they were of the same party, is one of the oldest tricks played by new prime ministers. John Major extracted a lot of mileage out of not being Margaret Thatcher. Gordon Brown did well for a short while by not being Tony Blair. Mrs May has scored some quick, easy hits by striking contrasts with Mr Cameron. She has – at least in words, we will see about the reality – junked the austerity agenda. She has talked some one-nation language about social reform. Many of her statements since she moved into Number 10 have been implicit, and sometimes explicit, rebukes of the Cameron government for being too posh, too privileged, too metropolitan and too obsessed with what she likes to disparage as “political games”. Her people often refer to the Cameron cabinet as “the previous regime” as if it was a government of a different party and conveniently forgetting that Mrs May sat in that cabinet for six years.

Nick Clegg’s forthcoming memoir is not friendly – he accuses her of misrepresenting immigration statistics – but the more wounding revelations are probably those at the expense of David Cameron and George Osborne. The former deputy prime minister gives an example of their games-playing when he reports that they weren’t interested in building more social housing on the grounds that it “just creates Labour voters”. Mrs May has also been helped because the vote to leave the European Union has not had the immediately traumatic economic consequences that some forecast. This doesn’t tell us anything about the long-term impact, but it has been a short-term fillip for the government. The bounce on the stock market has been accompanied by a rally in the Tory party’s poll rating, which currently hovers around the 40 point mark, an improvement on their vote share in the 2015 election. Then there is the competition – or lack of it. Having achieved what it existed to get, Ukip is trying to work out its continuing, post-Farage point. The Lib Dems struggle to get their voice heard. Labour is going through its own special kind of existential hell. No wonder some of Mrs May’s senior colleagues are bewildered that she has ruled out an early election to capitalise on all these advantages while she still has them.

For when parliament returns this week it will be a reminder that her structural position is weak. The Conservatives have a working majority of just 17, which is smaller than the number of people she sacked from the government when she culled the Camborne circle. If there was a common denominator to her purge, it was that the sacked were allies of the previous prime minister or friends of the former chancellor. The dispossessed may have lost their ministries, but they are still MPs. They will bide their time and nurture their wrath, waiting to unleash it when Mrs May starts to trip and fumble.

She has so far defined herself more in words than deeds. Mrs May is a relatively blank canvas on which Tories of different persuasions can paint their hopes, their dreams and their fantasies. Trouble will start when she starts to put flesh on Mayism. We have had a couple of harbingers. One was the negative reaction to the decision to water down the government’s anti-obesity strategy. Another was the recent revelation that £3,150 will buy you into a “business day” at the Tory conference, Mrs May’s presence included in the price of the ticket. People make the point, no less potent for being obvious, that offering companies the chance to purchase access to the prime minister sits badly with her rhetorical flourishes about being on the side of the many, not “the privileged few”.

Labour’s travails are a mixed blessing for the Conservatives. A weak opposition encourages two dangerous traits of the Tory beast: arrogance and ill-discipline. When I recently suggested to one of Mrs May’s cabinet that the implosion of the Labour party was not good news for the Tories over the longer term because it would foment their own divisions, he replied: “I regret to say that I think you are completely accurate.”

Mrs May has a thing about discipline. As home secretary, she was not famous for being either a delegator or a sharer of information with colleagues. This style is clearly being imported into her Number 10. She has told civil servants that she wants a more formal style of decision-making. Unlike her predecessor, she will not make the first announcement of a parliamentary vote on a military intervention via Twitter. Number 10 has issued an edict around Whitehall that all media bids for interviews with ministers, and on whatever topic, however significant or inconsequential, must now be cleared through Downing Street.

This will frustrate the broadcasters, who have all those hours of airtime to fill, and is already generating consternation, annoyance and resistance in departments. One minister, speaking to us without clearance from Number 10, describes this control order as “completely ridiculous”. Mrs May is not the first prime minister to try this and I suspect that, like others before her, she will discover that it is just not achievable for Number 10 to try to impose this level of domination. Some will put it down to control-freakery. Others will see it as an angry reaction to the cabinet leaks and spats about Brexit over August. A third explanation is nerves. Mrs May and the new team at Number 10 sit atop a government with a slender majority, a cabinet with some extremely combustible egos and a party with deep divisions that cannot be long disguised by the meaningless mantra of “Brexit means Brexit”.

Mrs May has had just over 50 days in Number 10, during which she has done a deft job of obscuring her vulnerabilities by projecting herself as a fresh yet solid leader serenely stepping in to take charge. But she, more than anyone else, will understand how shaky is the ground beneath her feet, how uneasy lies the head that wears the crown. She’s also been around long enough to know that the one certain thing about honeymoons is that they come to an end.