LONDON: It is in the nature of politics that within minutes of a party leader resigning, the minds of MPs turn to who the successor should be and, in particular, the pressing question of whether it should be them.
In pre-digital times, it was the installation of a dozen telephone lines into a small Westminster pad that was a sure sign of a leadership hopeful gearing up for action. Now the mass of text messages and mobile calls goes undetected, but you can rest assured that the number of phone conversations between MPs this weekend was at least 10 times the normal level.
Each MP is besieged with calls, testing, probing, recruiting, as the candidates try to assemble a credible team of followers or decide, with as much grace as they can muster, to bestow their support on another. Four days after David Cameron announced his departure and months before the final ballot, a candidate who is not ready to run in the next few days has no chance. At moments like this, politics is fast and brutal.
This much is familiar, but the circumstances are not. Last week's referendum has ousted a successful prime minister who only a year ago won a famous election victory. It has prompted Labour MPs to now confront the impossible position in which their own recent leadership election placed them. And it means the new occupant of 10 Downing Street faces a set of challenges more complex than any of their predecessors have encountered since the Second World War. Nothing less than extraordinary leadership will be sufficient.
How are the 330 Conservative MPs - and ultimately the 125,000 or so party members - to choose the new prime minister? There will be several candidates who have the experience, credibility and knowledge to do the job. But the question must be: do they have the clarity of vision to provide a divided and uncertain country with the confidence it needs, and do they have the strength of character to realise that vision in the teeth of pressures that could tear apart their party and their country?
A new prime minister will need to have a clear plan the day he or she is elected, and a decisive mandate for it from their party. The moment they arrive in the Cabinet Room, foreign leaders, major investors and most of the British people will want to know exactly what they are planning to do. The Conservative conference will be imminent. There will be no time to muddle through. This means each candidate should be expected to set out how he or she will deal with four dimensions of the immense questions facing the United Kingdom.
The first is to answer the question the Leave campaign most conspicuously refused to address: what is the relationship with Europe we are now aiming for? It will be difficult to cut any new advantageous deal with the EU, but if we don't know what we want it will be totally impossible. Are we open to joining the European Economic Area, along with Norway and Iceland, which would mean ditching the commitment to control immigration - or are we putting migration controls first and taking the economic consequences of that? The time for avoiding this question is over.
The second is the related challenge of giving businesses confidence to invest in the UK, or to think again about relocating their operations abroad. Many of us argued that the disadvantages of leaving would outweigh the advantages, but now the decision is made, the worst of all worlds would be to suffer those disadvantages without exploiting all possible advantages. The candidates need to say how they would use "taking back control" to make Britain a good long-term bet. Show how taxes on enterprise can be cut steadily for a decade, pensions and saving simplified, and EU regulations abandoned where they are too burdensome or counter-productive. With the City in a quandary as to what to do, consider adopting US-style financial regulation instead of the EU model.
Whichever side of the referendum the new PM was on, they need a lot of the people who voted Remain to be excited and inspired by the programme of a government that is now committed to Leave. A serious but radical economic programme should be at the heart of that.
The third dimension is scarcely less vital: having a plan for keeping the United Kingdom together. The immediate tactics of the Scottish nationalists, aimed at exacerbating divisions between London and Edinburgh, were foreshadowed in what I wrote last week. Those who led the Leave campaign showed an inexcusable complacency about the future integrity of the UK itself, ignoring all concerns about Scottish independence, the delicate position of Northern Ireland, and the exposed position of Gibraltar. The new prime minister must be someone who has shown they have a fighting chance of saving a Union that is now in critical danger.
The final dimension is a more partisan requirement, but still vital for all of us who want to see the gains this country has made in recent years safeguarded for the future. The next Tory leader has to be able to keep the Labour Party pushed to the fringes of politics, with a style of Conservatism that combines economic credibility with a modern social liberalism to dominate the centre ground of political debate.
Today's (Monday's) convulsions in the Labour leadership are spectacular to behold. Their party could formally divide, or turn in on itself in a new and bitter leadership election of their own. But Tories would be wise not to write off a party with such deep roots, that might still reinvent itself with more credible leadership or reap the benefits of this new age of rebellion and discontent. This means a strong domestic agenda for Conservatives, building on the Cameron years and appealing to the people who feel abandoned by globalisation and technological change. People who are low paid, or run small businesses, or live in depressed towns, need to know that ministers in Whitehall understand their problems and can help.
A leader who can satisfy these needs is rare indeed.
So my advice to the MPs whose phones are now ringing red hot is this: it is no time to be nice to every candidate. If they don't know how to cope with these four great challenges, they shouldn't be our prime minister.