Having skilfully negotiated his way to power anyway, Cameron has been good at performaing the role of Prime Minister. His personal leadership skills - especially being calm under pressure in May 2010 as in October 2007 - have been good. But he has been lucky too, perhaps especially in how little attention has been paid to a record as a party leader as much about failure as success.
For 2010 was also the year in which just about everybody overestimated David Cameron.
If you want to understand the biggest political mistake which each of the major parties made during this turbulent political year, look back and you will find that is the common thread. Each party made a crucial miscalculation about the political context, largely because they over-estimated the electoral power of David Cameron.
Why were the Conservatives the first election frontrunners to take the (laudable) risk of agreeing to televised leaders' debates? Surely because they were very confident that the superior communication skills of the leader would see the gamble pay off handsomely. Every Conservative expected their man to rise best to the occasion and finally "seal the deal" with the voters. He didn't. So Michael Ashcroft leads a vocal camp of those who believe the debates cost the Tories a majority.
Why were Labour so much the worst prepared of the three major parties for Coalition talks which represented the governing parties best conceivable election outcome? It was because Labour expected Cameron to win. The sad fact is that a hung Parliament in which Labour might still be able to negotiate a further spell in office exceeded the expectations of the party's campaign strategists. (John Rentoul argued cogently last weekend that the Labour leadership question remained open up to the failed January 6th plot (I thought at the time that June 2009 had been largely decisive). What can not be disputed is that Gordon Brown was bolstered in both 2009 and 2010 by the belief that any chance to stay in power was unsalvageable. Cabinet ministers who were personally sure Brown would not turn things around were also too fatalistic to believe any move against him worth the risks of change).
Thirdly, there is only way to make sense of why Nick Clegg made so much in the campaign of a tuition fees pledge he could not keep, which has both done the most and most symbolised the deep damage to the LibDems' public credibility and popular standing. Give Clegg his premise that he expected to fact a Tory government, and the pledge would demonstrate fox-like political cunning from which the LibDems could have benefitted electorally.
Over-estimating David Cameron proved the national sport of the political classes in 2010. It united the bookies and the punters: Betfair projected a 62% chance of a Tory overall majority on the day that the campaign proper began. It united the newspaper propreitors, editors and much of the blogosphere too. If Iain Dale's blogging is much missed, that is not because of the accuracy of his political crystal ball. Even after the polls closed, Dale was offering to "run stark naked bollock down Whitehall" if the (uncannily accurate) exit poll was right.
There were exceptions. The New Statesman unfashionably predicted a hung Parliament. The Tory right was sceptical of the Cameron strategy, though mostly kept quiet until after the event when this was reflected in Tim Montgomerie's influential post-election inquest for ConservativeHome, and Spectator editor Fraser Nelson's observation that "“It is odd to think of David Cameron as the most electorally unsuccessful Tory prime minister in history”.
To give credit where it is due, the most significant group which was clear sighted in not falling for this general exaggeration of Cameron's popular appeal was the Tory leadership itself.
They knew on the morning of the election that they had failed to secure a majority. Andrew Cooper has said that Cameron spent election night focused not on 326 seats but whether they would get across the 300 seat line, and so probably get in. They had known for weeks and months that no Tory positive argument had any grip with the voters they needed to persuade - yet continued to return to the comfort zone of negative attacks on Labour, while secretly planning for hung Parliament negotiations.
The desire for a change from Labour was remarkably high - 74% no less. The number who wanted a change to the Tories (34%) or the vote they got in the end (36%) was astonishingly low in the political conditions of 2010. The political (expenses v sleaze) and economic conditions (recession v recovery) were as favourable for the opposition than in 1997 - and arguably better. Then Tony Blair stormed home, putting 9% on Labour's share. Cameron fell far short of the campaign he sought to emulate.
How badly did it go wrong? The most interesting thing about recent British electoral history is that the Tories got 31-32% in 1997 and 2001 - suggesting a very strong 'tribal' Tory vote in any circumstances - and yet only 36% in 2010.
The Tories got 33% in 2005 with Michael Howard, before the recession, and without facing Gordon Brown. Does anybody think David Davis - or even Michael Howard, promoting his bright young things like Cameron, Osborne and Gove - would not have got very close to 36% in 2010?
So was David Cameron's "reaching out" appeal and all of that brand detoxification, in the end, worth any votes at all? Or has his party leadership failed in the core test he set himself, meaning he had to sneak into Downing Street by default?
Making the same mistakes: How everybody is over-estimating the Tories again
If over-estimating David Cameron skewed most pre-election commentary, what have we all learnt?
Nothing, it would seem.
The funny thing is that all of the same people are doing it again. Matthew d'Ancona even declared that the Tories had already won the next election when Labour elected Ed Miliband. Shouldn't he first offer a coherent explanation of why they failed to win the last one against Gordon Brown?
Over-estimating David Cameron has skewed much commentary of the political context after the May election. Take the widespread assumption that Cameron was pretty certain to quickly win a clear majority if there had not been a Coalition in May. Nobody knows what would have happened in a second election - but there is little or no evidence for this belief.
(Those who argue that was very likely have never answered the central mystery. If a second election would have seen the Tories romp home, why on earth didn't they engineer one? Why was there no significant pressure from the Tory shadow Cabinet or Parliamentary party to hold out for this inevitable prize? Surely because it was very far from a sure thing. Political history suggests asking the same question again gets a similar result - with the Tories then having the mandate problem, perhaps snatching opposition from the jaws of government).
But, again, there is one group who doesn't agree. Yes - it's the Tory leadership again. Think about why they are yet again cranking up those electoral pact missives to their LibDem beau Clegg, with a new round sparked by Ben Brogan's authoritative account during the tuition fees debacle.
Yet everybody seems to buy the argument that it is a "save Clegg" campaign (from the people who brought you "Kill Clegg" with their press friends last time).
Of course, this makes absolutely no sense.
Why would Tories care one iota what happens to Nick Clegg if, amidst LibDem collapse, they could simply strike out and when a majority of their own amidst those sunny economic uplands? (ConservativeHome can prove that four out of five don't).
So remember this in 2011: All of the pact talk is surely about saving Tory skins, not LibDem ones.
There's a simple reason too. The Tory leadership can see why the next election could well be rather harder to win than the last one. Confident Tory ideologues like Tim Montgomerie can put these deep insecurities down to the status of the Philip Gould-bible in ProgCon circles, emulating the insecurities of New Labour which meant Tony Blair and Cherie feared defeat by William Hague in 2001.
The difference is that the evidence of Tory electoral weakness is much stronger.
If the Tories want a majority, they have to break that 36% glass ceiling.
So who wasn't convinced the Tories had changed in 2010?
These key groups of voters did not believe Cameron when he said that he had changed the Tories. Specifically, Scots, northerners, public sector workers, ethnic minorities and Londoners - even when fed up with Labour - found it very difficult to vote Tory.
Many thought it was time for a change. But they didn't believe David Cameron's promise that he could eliminate the deficit with no cuts to frontline services! Even with a personal pledge from a PM-to-be, they thought it sounded like a fairytale.
Why? As Philip Cowley and Dennis Kavanagh report on Populus' focus groups for the Tories.
The most worrying finding for the Conservatives was the perception that they would, in a crunch, stick up for rich and privileged people. Cameron privately confessed late in 2008 that the persistence of this last image kept him awake at night.
So how is that now going with those who did not vote Tory in 2010?
We know from the government's falling approval rating that those who voted Tory are pretty happy with the government - and most of those who didn't are not.
Note now how the government wins praise for its bold moves - reckless for some in transforming education, health and welfare entirely while undertaking one of the greatest gambles with the economy and public spending - almost always from people who were cheering it on in May 2010.
Look at the reactions of those who didn't - including those anxious LibDem voters - and it looks harder to see how the Tories will be able to persuade those voters that the Coalition has shown that they have laid to rest all of those "same old Tories" attitudes after all.
Labour has a lot of work to do to try to get elected - on economic credibility, and on putting together a different but electorally effective political and policy programme, rather than running on essentially the same platform for a fifth time when the agenda is 18 years old. But it is simply a misreading of the political context to regard Labour as facing the scale of electability challenge which Labour faced from 1980-87, and with hindsight through to 1992 too, or the Tories from 1997 until 2005
There's a long way to go in this Parliament. Either major party has a lot of work to do to seek an overall majority, as Labour uses the defection of one-third of LibDems to start neck-and-neck, and the Tories work out how to break that 2010 ceiling without which they risk remaining short of a serious shot at a majority.
Yet media commentators prefer to magnify backbench grumblings than to think seriously about the politics of the next election.
For example, what if David Cameron asks the country for a Tory majority mandate - and comes back without one?
Surely now, the legitimacy problem would be his. He's Ted Heath in 1974. He's Gordon Brown in 2010.
If he has less votes or seats than last time, he should surely announce he is taking responsibility and get off the stage pronto, to see if his party can hold on under somebody else. If the Commons arithmetic means another combination is viable, the legitimacy issues would be completely different from May 2010.
Should Labour have one more seat or one more vote, they have every right to the first talks with any minor parties who survive!
The Labour leader has a perfectly good shot at becoming PM, even before he has done a great deal to set out his stall as party leader. He has the problem of every political leader in receiving contradictory advice - to set out his stall in 100 days while understanding that nobody wants to hear from a defeated party yet. So it is fair to say that Ed Miliband has yet to give his party clear definition; it is arguable that David Cameron's own public definition is not much clearer - after five years, not three months - and indeed that the increasingly entrenched public definition of his government (that its basically about cuts) is quite at odds with what he wants it to be.
If I were Ed Miliband, there are two lessons I would take from David Cameron's party leadership so far.
Firstly, that credibility can't be won by counter-intuitive photo opportunities. Cameron had a bold and positive first 100 days. He got his party a hearing. It was just the next four and a half years that were wasted, leaving voters ultimately pretty unsure as to what, if anything, Cameron really had to say.
Secondly that, well before the end of the Parliament, Ed Miliband had better have come up with a "doorstep narrative" for the next election campaign which is a damned sight more useful than "the big Society" was for the Tories at the last one.
We shall see whether Iain Martin gains more recruits for his Don't Underestimate Ed Miliband campaign. Observe that there are two secret founder members - David Cameron and George Osborne.
One sometimes hears the accusation that the Tories approach to power suggests that they think they are born to rule misses the mark. They are much less certain than they appear. The bigger problem is with the rest of us - and how our deference in expecting to see the Tories govern distorts political perceptions.
That helped to make 2010 David Cameron's year. Will he stay as lucky in 2011?