How late did the prime minister decide to stay awake till on Sunday night, as Ukip triumphed in the Euro elections? Was he up for the humiliating defeat of his fellow Tory Martin Callanan, chairman of the European Conservatives and Reformists (ECR) group in the European Parliament? Did he hear Nigel Farage refer to the European election results as an "earthquake" because "never before in the history of British politics has a party seen to be an insurgent party ever topped the polls in a national election"?
And did he perhaps pause to consider, as he eventually hit the sack inside Downing Street, how the rise and rise of Ukip could cost him a second term as prime minister?
Pity the poor PM. In recent years, aided and abetted by his hard-right Australian strategist Lynton Crosby, master of the 'dog whistle', Cameron has tried a range of tactics to try and lance the Ukip boil: from insulting to appeasing; from imitating to ignoring.
None of them have worked.
Cameron tried belittling and mocking his right-wing, anti-Europe rivals. In 2006, the then leader of the opposition famously dismissed Ukip as a "bunch of.. fruitcakes, loonies and closet racists".
By 2013, however, with Ukip grabbing a whopping 23% of the vote at the local elections, the PM had been forced to change his tune: "We need to show respect for people who have taken the choice to support this party and we are going to work really hard to win them back."
Cameron tried seeing off Ukip with his own attempt at hardcore euroscepticism. Remember his so-called 'Bloomberg speech', in January 2013, in which he pledged an 'in/out' referendum on Britain's membership of the EU by the end of 2017? The speech that was supposed to have shot Ukip's fox? "The announcement," in the words of the prime minister's favourite columnist Dan Hodges, "that in all likelihood will see him returned to power"?
Fast forward 17 months and the PM now has influential and media-savvy Tory backbenchers, such as David Davis and Adam Afriyie, snapping at his heels, echoing Ukip's demand to bring forward the date of his in/out referendum. His chief lieutenant, the chancellor George Osborne, conceded on Saturday that the public did not "believe" the Conservatives would hold a referendum on Britain's membership of the EU.
Cameron tried stealing Ukip's anti-immigration clothes. His cabinet ministers railed against benefit tourism and health tourism by EU migrants; his Home Office sent vans out into ethnically-mixed areas of London to tell illegal immigrants to 'go home' (a move described by Farage himself as "nasty").
Net migration has fallen on Cameron's watch - yet Ukip, and not the Tories, remain the most trusted political party on the toxic issue of immigration. In fact, while three out of four voters say they support the PM's stated aim of reducing immigration to the "tens of thousands", only one in ten think it is likely he will achieve it. As Ed Miliband put it, in his response to the Queen's speech in May 2013, "The lesson for the prime minister is you can't out-Farage Farage."
So what now? How do the Tories tackle the threat posed by Farage and his merry men? What happens if Ukip, against the odds, triumph in Tory-held Newark on 5 June? Will Conservative Party high command continue to persist with its message of 'Vote Farage, Get Miliband'?
Let's be clear: Ukip isn't going to win the next general election. It isn't on course to form a coalition government with the Conservatives - the truth is that it is unlikely to win a single Commons seat come May 2015 (despite a report in Sunday's Observer that the party plans to "target at least 20 parliamentary seats at the next general election"). As the New Statesman's George Eaton tweeted, "Voters wouldn't let UKIP near the NHS."
But the Farageists will win plenty of votes up and down the country next May and could very well affect the outcome of the general election. In 2009, Ukip secured 16.5% of the vote in the European elections, yet just 12 months later could only win 3.1% of the vote in the general election. This time round, however, research by the British Election Study suggests at least half of the voters who backed the anti-Europe party in this year's Euro elections plan to stick with Ukip in the general election.
This should be the stuff of Cameron's nightmares. Despite claims to the contrary from academics Rob Ford and Matthew Goodwin, as well as Labour figures such as policy chief Jon Cruddas and former home secretary Jacqui Smith, the truth is that Ukip doesn't pose as much of a threat to Labour as it does to the Tories. It is Cameron's Conservatives who are taking the biggest beating from the Farageists - which explains why it is Tory, and not Labour, MPs calling for a pact with the anti-Brussels party.
Consider the poll of over 4,000 people commissioned by Lord Ashcroft, the Tory peer, which revealed that more than half of Ukip voters backed the Tories in 2010, compared to only 15% of Ukippers who said they'd voted Labour at the last election.
Consider also the results of the local election results on Thursday: "UKIP utterly failed to make any inroads at all in big Labour cities, returning absolutely zero councilors in Manchester, Liverpool, Birmingham, Newcastle or Leeds," noted blogger and analyst Thomas G. Clark.
As the Guardian's Martin Kettle observed back in September 2012, even Ukip's miniscule 3.1% vote share in 2010 "may have made some of the difference between a hung parliament and an outright Tory win. Small though it was, the Ukip vote exceeded the majority in 21 marginals the Tories failed to win in 2010, including Ed Balls's seat in West Yorkshire." Imagine, then, the impact on the Tories next year if Ukip win eight, nine, even 10%, in a general election which, by common consensus, will be even closer than the 2010 contest.
Farage could turn out to be the Ross Perot of British politics - splitting the anti-Europe, centre-right vote and thereby handing victory to the pro-Europe, centre-left.
Lucky Ed. Poor Dave.
Despite the fact that, for once, it has the advantage of being 100% correct.