All political leaders sooner or later have to confront what may be called, for this extraordinary week at least, the David Davis test.

The proud powers-that-be have their grids, timetables and agendas which they seek to impose upon us. But then some unforeseen eruption from nowhere blows apart their carefully laid plans and they are forced to find their way without a route map. How Gordon Brown and David Cameron meet their David Davis tests will reveal their true mettle.

In the wake of his shadow home secretary’s unheralded resignation, it was the Conservative leader who was initially in deadlier peril. Brown, after all, had won his vote in the Commons on 42-day pre-trial detention for terrorists. Commentators described it as a “Pyrrhic victory”. Well, that’s a lot better than a Pyrrhic defeat. But now it is the prime minister who faces his David Davis test thanks to the crushing Irish no to Europe.

Last Friday Cameron was supposed to be advancing his agenda of social entrepreneurship at the Eden Project in Cornwall, the latest in a series of set-piece speeches aimed to hone his message. As I wrote last week, the opposition leader was to bind together his themes of improved schools, welfare reform, family policy and a beefed-up voluntary sector into a coherent programme for government. Buoyed up by 20-point leads in the polls, his shadow cabinet has been working with civil servants and businessmen, under the direction of former minister Francis Maude, to hit the ground running after a general election victory.

It is but a small step from grand talk of power to flying by the seat of your pants. Cameron was soon reeling from the surprise resignation of Davis to fight a by-election in his constituency to highlight the erosion of civil liberties. The Tory leader had already backed Davis on 42 days and had come out in favour of abolishing ID cards, too, convinced that his former rival for the leadership had the better of the argument against hawkish colleagues. Doubtless, calculation played a part: Liberal Democrat and Labour voters in southern marginals seats like a “liberal Tory”.

Davis has now chosen to go his own way as a campaigner for civil rights. The broken-nosed former member of the territorial SAS is a stormy petrel of politics, combining principle, pugnacity and sheer devilment in equal measure. This makes him good company for journalists, not Conservative party leaders. His replacement and ally Dominic Grieve lacks his common touch.

The rejection by the Irish people of the revised Lisbon consti-treaty also appears to resurrect the dread subject of Europe. Does Cameron want to “lead a crusade” on either of these issues beyond the point of making his position clear? Is Wayne Rooney an old Etonian, you might just as well ask. As one rare pro-European in his team once told me: “He forgives me my views as long as I shut up about them. But he doesn’t like hearing much from the other lot either.”

The last thing the Conservative leader needs is to sound like a single-issue-obsessed civil libertarian or Eurosceptic. That is not to decry either cause. It would take a heart of stone not to laugh as the people of Ireland blew a collective raspberry at Brussels, while even her opponents admit that Shami Chakrabati of Liberty has led a spirited campaign against 42 days and that she has found a doughty ally in Davis. But a leader knows that he can’t afford to let single issues take over the message.

Most floating voters make their choice on tone of voice and language. It’s over in a second for them. What has this politician got to say to me? Is he addressing my concerns or those of a few fanatical activists? Is he in it for himself or does he genuinely care about my school, my hospital, my job and my mortgage? This is what ultimately swings an election.

The majority of voters dislike Brussels. They are also prepared to lock up terrorists and throw away the key without trial: habeas corpus, schmorpus. But as previous Conservative leaders have found to their cost, people are unlikely to change their votes over Europe. And unless Al-Qaeda commits a particularly foul atrocity some time soon, Brown will not have won many new friends for Labour but will have lost a few old ones.

The Cameroons are now crowing that the Irish have thrown them a lifeline. Their greatest dilemma was what to say on Europe in a general election had a yes verdict been delivered in Dublin. If they had stuck to their promise of offering a referendum on a treaty ratified by Britain, the government would have jeered that the Tories would have to get the permission of 26 other reluctant countries to unpick it. With one bound, or rather an Irish jig, the Tories rejoice that they are free.

As one rising Cameroon put it to me in happy rapture at how the wheel of fortune had turned yet again: “George Sampson won Britain’s Got Talent a few days ago, Gordon Brown won Big Brother on Wednesday, David Davis was chosen as the Apprentice on Thursday and the Eurovision contest crown was taken by the Irish on Friday. It’s a crowded schedule, there’s always some new contest that takes attention from the old one.”

These past 11 months Cameron has been doing well to present himself sympathetically while Brown has been shedding many natural Labour supporters by sounding detached and uncaring. The government’s cavalier abolition of the 10p tax band and subsequent recantation under pressure from a backbench revolt is a graphic example. The working poor at the Crewe by-election thought that the belated moves to alleviate the unfairness were made to ease the prime minister’s pain, not their own.

Brown has also tried a tough-guy solution to his image problems by “taking a stand” on 42 days. Admirers of Armando Iannucci’s brutal BBC satire of spin and new Labour life, The Thick of It, will have noted how life imitates art. Try, for instance, putting this subversive dialogue into the mouths of the prime minister and home secretary: “We’re looking for a popular punishment that won’t cost a penny.” “Hanging?” “You’re joking! . . . You are joking?”

The prime minister got away with 42 days, as I predicted, on the back of support from the Democratic Unionists and Muslim Labour MPs. His strategy seemed vindicated, at last, when Davis pulled off his coup de théâtre. Brown quickly presented the opposition as divided and a bit loopy. “Enoch Powell at least got the dockers out in support in 1968 when he made his stand on the Rivers of Blood speech, agrees a Tory critic. “Who is going to march for Davis?” If no Labour candidate stands against him, the field is left open to the Monster Raving Loonies and other eccentrics. It’s “a farce”, said Brown, though at least one of his rebel MPs is lending Davis support.

However, it is the prime minister who finds himself as lead actor in another farce: Europe. Parliamentary ratification of the European treaty will proceed despite the no vote in Dublin. Never mind that the whole exercise has been invalidated by the Irish people, never mind Labour’s manifesto commitment to hold a referendum on the constitution. Brown has it in his power to ditch the whole process. It would be wise. It would show that he could pass a David Davis test.

Our poll today shows the Tory lead lengthening despite the Davis resignation. This week the Bank of England governor is expected to write a letter to the chancellor explaining why inflation has risen above 3% and those nice friends of Mr Brown in Brussels will be ticking him off for putting the nation’s finances in the red. Out there people are grumpy and a bit scared.

The facts remain the same. The prime minister is unpopular. The Tory lead is massive but shallow (the voters are not convinced yet that they will do much better in government).

Europe and the balance struck between civil liberties and national security are, of course, of great moment. But Brown and Cameron know that the David Davis test is trumped in the voters’ minds by another supreme one: what are you going to do for me?

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