Now the Tories have the 48 signatures to oust Theresa May, Brexit itself hangs on a knife edge

The prime minister hoped her move would buy her some time. It didn’t – only 24 hours. In that period, she made little progress in her last-ditch talks with fellow EU leaders

“One vote is enough,” David Cameron advised Theresa May during a previous frenzy of speculation that she would face a vote of confidence as Conservative Party leader. They were discussing the winning margin needed for a leader to be able to carry on after such a vote.

Now the endless speculation is over and tonight we will have a real contest, after the required 48 Tory MPs (15 per cent of the total) demanded one. The prime minister will certainly try to hang on even if she merely scrapes home; she would see it as her duty to get Brexit “over the line”.

But could May win and lose at the same time? If she manages only a narrow victory, the position of the cabinet would be crucial. It has rallied behind May today. But that could change after a pyrrhic victory. As I recalled last week it was the cabinet rather than Tory backbenchers who ended Margaret Thatcher’s premiership in 1990.

The timing of the vote is not great for May. She must be kicking herself for failing to pre-empt today’s challenge last month when the hardline Brexiteers in the European Research Group (ERG) threatened one.

John Major resigned as Tory leader (but not prime minister) in 1995 to face down his critics. May would have won if she had done the same a few weeks ago. But the result could be closer now. Her authority, and trust in her amongst her MPs, has drained since her humiliating retreat in delaying yesterday’s scheduled Commons vote on her Brexit deal. She hoped her move would buy her some time. It didn’t – only 24 hours. In that period, she made little progress in her last-ditch talks with fellow EU leaders to try to soften the backstop to prevent a hard Irish border.

Tory MPs expect warm words that do not change the withdrawal agreement, which will not help May’s chances in the vote.

May escaped one crunch vote but now faces another. The difference between them could prove critical.

The delayed Commons vote was public; a minister who could not back May’s deal would have to resign. The Tory party confidence vote is a secret ballot. Which means that ministers and backbenchers can pledge public loyalty to May and then vote against her.

The lying game brought down Thatcher. Her complacent campaign team thought they had enough MPs to defeat Michael Heseltine’s challenge – the rules were different then – but she failed to win by the necessary margin and resigned after cabinet ministers told her she would lose to him in round two.

A third of Major’s MPs either voted for his challenger John Redwood or abstained in 1995. Yet he carried on. That might provide some cover for May if 100 of her MPs vote against her tonight – perfectly possible since 110 have criticised her Brexit deal. Her target number is 158 (a simple majority of Tory MPs) but some of them believe she would be vulnerable if her opponents secure more than 100 votes.

Her allies insist the timing could work to May’s advantage. Her ERG enemies are arguing amongst themselves about who their candidate would be in the leadership election that would take place (without May) if she loses tonight.

Ousting May when she is due to fight for a better deal at tomorrow’s EU summit will be seen by some Tories as unpatriotic.

Some, including the 30 pro-EU Tory MPs, will hold on to Nurse Theresa for fear of something worse – such as a Brexiteer prepared to crash out of the EU without a deal in March, or one indulging in what Brussels calls the “magical thinking” that technology not yet invented can prevent a hard Irish border.

In the event of a contest, the most pro-Brexit candidate would have a very strong chance of winning.

Tory MPs would choose a shortlist of two in a series of ballots but the final decision then lies with the Tories’ 120,000 grassroots members, whose instincts are Eurosceptic.

That’s why Sajid Javid and Jeremy Hunt, who both backed Remain in 2016, are now born again Brexiteers who flirt with the idea of a “managed no deal.” While professing loyalty to May, of course.

Tory party rules could work against May. If she wins, she could not be challenged for another 12 months. Many Tory MPs want her to stand down soon after the UK’s scheduled departure date in March. A win would allow May to shape the talks on a UK-EU trade deal starting in April. The MPs know there is a prospect that the Brexit crisis results in a general election, and do not want May to lead the party into another one after her disaster last year.

Although many Tory MPs have no love or even respect for May, they know that if they remove her, the same parliamentary arithmetic would greet her successor. No majority for May’s deal, a no-deal exit, a Final Say referendum (yet), a Canada or Norway-style deal. Although everything would have changed, nothing would have too.