In the run-up to the 2015 election one of the handicaps David Cameron had to finesse was the fact that net migration to the UK was three times as high as he had promised it would be. Remarkably, none of the opprobrium this failure provoked brought forth the name of Theresa May, the cabinet minister actually entrusted with bringing migration down. Then, as now, it was as if the icy Home Secretary had a dark magic that warded off all critical scrutiny.

The fact that her lead role in this fiasco went unmentioned reflects Mrs May’s clever, all-consuming efforts to burnish her image with a view to become prime minister. After all, Mrs May’s tenure as Home Secretary has been notably unsuccessful. Its abundant failures include a succession of derelictions that have left Britain’s borders and coastline at least as insecure as they were in 2010, and which mean that British governments still rely on guesswork to estimate how many people enter and leave the country.

People find this hard to credit because she exudes determination. Compared to many of her cabinet colleagues she has real gravitas. And few who follow British politics would deny that she is a deadly political infighter. Indeed Theresa May is to Westminster what Cersei Lannister is to Westeros in Game of Thrones: no one who challenges her survives unscarred; the welfare of her realm is a much lower priority than her craving for power.

Take the UK Border Force. Despite the increased terror threat, it was already a dangerously underfunded and demoralised agency when Mrs May announced in April that its budget was to be cut. Then in May, after two people-smugglers’ vessels were found sinking off the Kent coast, the public discovered that the Force has only three cutters protecting 7,700 miles of coastline. Italy by contrast has 600 boats patrolling its 4722 miles.

Considering the impression Mrs May gives of being serious about security, it’s astonishing that she has also allowed the UK’s small airfields to go unpatrolled — despite their attraction for traffickers of people, drugs and arms, and the urgings of the security services.

Then there is the failure to establish exit checks at all the country’s airports and ports. These were supposed to be in place by March 2015.

Unfortunately the Border Force isn’t the only organisation under Mrs May’s control that is manifestly unfit for purpose. Recent years have seen a cavalcade of Home Office decisions about visas and deportations that suggest a department with a bizarre sense of the national interest. The most infamous episode was the refusal of visas to Afghan interpreters who served with the British forces in Afghanistan – as Lord Guthrie said, a national shame. Mrs May has kept so quiet about this and other scandals – such as the collapse of the E-borders IT system, at cost of almost a billion pounds – that you might imagine someone else was in charge the Home Office.

It is not just a matter of the odd error. Yvette Cooper pointed out in 2013 that despite Coalition rhetoric, the number of people refused entry to the UK had dropped by 50%, the backlog of finding failed asylum seekers had gone up and the number of illegal immigrants deported had gone down. You’d almost imagine that Mrs May was so busy defending her turf and polishing her image that there was no time left to embark on the major reforms her department obviously needs.

The reputation for effectiveness that Mrs May enjoys mostly derives from a single, endlessly cited event: the occasion in 2014 when she delivered some harsh truths to a conference of the Police Federation. Unfortunately this was an isolated incident that, given the lack of any subsequent (or previous) effort at police reform, seems to have been intended mainly for public consumption.

In general Mrs May has avoided taking on the most serious institutional problems that afflict British policing. These include, among other things, a disturbing willingness by some forces to let public relations concerns determine their policing priorities, widespread overreliance on CCTV, a common propensity to massage crime numbers, the extreme risk aversion manifested during the London riots, and the preference for diverting police resources to patrol social media rather than the country’s streets.

There is also little evidence that Mrs May has paid much attention to the failure of several forces to protect vulnerable girls from the ethnically-motivated sexual predation seen in Rotherham and elsewhere. Nor, despite her proclaimed feminism, has Mrs May done much to ensure that the authorities protect girls from certain ethnic groups from forced marriage and genital mutilation. But again, Mrs May has managed to evade criticism for this.

When considering her suitability for party leadership, it’s also worth remembering Mrs May’s notorious “lack of collegiality”. David Laws’ memoirs paint a vivid picture of a secretive, rigid, controlling, even vengeful minister, so unpleasant to colleagues that a dread of meetings with her was something that cabinet members from both parties could bond over.

Unsurprisingly, Mrs May’s overwhelming concern with taking credit and deflecting blame made for a difficult working relationship with her department, just as her propensity for briefing the press against cabinet colleagues made her its most disliked member in two successive governments.

It is possible that Mrs May’s intimidating ruthlessness could make her the right person to negotiate with EU leaders. However, there’s little in her record to suggest she possesses either strong negotiation skills or the ability to win allies among other leaders.

It’s surely about time – and not too late – for conservatives to look behind Mrs May’s carefully-wrought image and consider if she really is the right person to lead the party and the country. There’s a vast gulf between being effective in office, and being effective at promoting yourself; it’s not one that Theresa May has yet crossed.