Will Ed Miliband's lead over David Cameron last? YouGov President Peter Kellner looks to history to judge our latest results

It may not last.

Short-term blips should not be confused with long-term trends. In polling, as with gravity, what zooms up generally drops back down. Nevertheless, something noteworthy happened last week, and it might portend a closer battle at the next general election than I expected just a few weeks ago.

In YouGov’s latest survey for the Sunday Times, Ed Miliband has overtaken David Cameron as the most liked – or, more accurately, least disliked – of Britain’s three main party leaders. Thirty-two per cent now say he is doing well, while 55% say he is doing badly. In net terms (% saying ‘well’ minus % saying ‘badly’) his rating his now minus 23. Cameron’s rating is now minus 29, and Nick Clegg’s minus 54. This is the first time since the height of the phone-hacking row last summer that Miliband has been ahead; and his lead over Cameron is his highest since his first few honeymoon weeks as party leader.

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How different things are to when I last assessed the party leaders back in January.

Then, Miliband was trailing Cameron badly. I pointed out that no opposition leader with such dire poll ratings in the past forty years had ever gone on to become Prime Minister. That position remained much the same up to the March Budget.

But since then, we have had spats over the granny tax, pasty tax, cut in top tax rate, threatened fuel strike, return to recession, ministers' relations with the Murdoch empire, the bungle over the extradition of Abu Qatada, the return to recession, and rumbles among some Conservative MPs and Tory-supporting papers that Cameron is not promoting his party’s basic values forcefully enough.

We have also had the local elections, which, with the exception of Ken Livingstone’s defeat in London, brought good news for Labour in every corner of Britain.

This is how the net ratings of the three leaders have moved since the start of the year:

Cameron Miliband Clegg January 5-6 -7 -46 -49 March 15-16 (just before Budget) -5 -45 -46 March 30-31 (at height of post-Budget rows) -27 -41 -53 April 26-27 -31 -38 -58 May 10-11 -29 -23 -54

As those figures show, nothing much moved until the Budget. Since then, there have been three distinct phases to the change in fortunes.

Phase one: In the days after the Budget, Cameron’s ratings declined sharply while there were only minor changes to the ratings of Miliband (up a bit) or Clegg (down a bit).

Phase two: Through April, little changed: Cameron’s and Clegg’s ratings got fractionally worse, and Miliband’s fractionally better.

Phase three: In the past fortnight, Cameron’s and Clegg’s ratings have stayed down, while Miliband’s have moved sharply up.

What will happen now? The next few weeks could be decisive. Cameron should be worried that the post-Budget dive in his ratings has now lasted six weeks. He has reason to fear that he is suffering not a short-term lovers’ tiff with the electorate but a more serious fracture in that relationship. The longer his poor ratings persist, the harder he will find it to regain voters’ respect.

As for Miliband, the sharp rise in his rating is too new for us, or him, to be sure it will persist. In the past, mid-term electoral success, in local or by-elections, has frequently produced short-term bounces for the winning party and its leader. I should not be surprised if Miliband’s rating slips at least part of the way back in the weeks ahead.

However, it is no longer hard to imagine that Cameron has lost his huge advantage over Miliband and that both men will attract broadly similar ratings in the months ahead. This could change the dynamics of the current Parliament and the prospects for the next election.

Lessons from history?

To see why, we need to revisit the central point I made in January, that, since 1970, opposition leaders as unpopular as Miliband have never before gone on to win the next general election. One reason for that is that, with just one exception, every unpopular opposition leader has faced a Prime Minister with either positive, or only fractionally negative, ratings. It was not just the unpopularity of the opposition leader, but the gulf between his reputation and the person he was trying to depose.

The exception was 1981. According to Gallup, Labour’s Michael Foot had dire ratings, dropping at one point to minus 51; he faced Margaret Thatcher whose own lowest rating that year, minus 45, was little better than Foot’s.

However, Thatcher had some colossal advantages: a hopelessly fractious Labour Party, a new centre party, the Social Democrats, dividing the progressive vote, and, in the year leading up to the 1983 election, victory in the Falklands and a rapidly recovering economy which made room for tax cuts. In the year leading up to that election, Thatcher was far more popular than Foot, and her party went on to win a landslide victory.

Cameron has little reason to expect rescue to appear in the shape of Labour infighting, military triumph or economic buoyancy, let alone all three. However, he does enjoy one big advantage: Labour still attracts much of the blame for Britain’s economic problems, and Cameron and George Osborne are still more trusted to safeguard the nation’s finances than Miliband and Ed Balls. If the Tories maintain that advantage, it could be crucial in a tight election.

But suppose the economy remains sluggish, and allows Labour to fight this particular issue to a draw in 2015. And suppose Cameron and Miliband have similar ratings for the next year or two.

History provides just one parallel, but we have to look further back into the mists of time, to the late Sixties.

Harold Wilson’s Labour government was in all sorts of trouble. It suffered the humiliation of having to devalue the pound. Wilson’s ratings were on the floor. Strikes were rife and the Cabinet often divided. But Edward Heath, the opposition Tory leader, was just as unpopular.

In the months leading up to the 1970 election, Wilson’s ratings improved. By election day he was much better liked than Heath. Yet Heath won: Wilson’s mid-term travails had been too profound, and his late surge too fragile, to give him the victory he expected.

History seldom repeats itself in a tidy manner; so I am not yet predicting that David Cameron will end up like a Tory Harold Wilson, or Ed Miliband a Labour Edward Heath. I still think a Conservative victory is the likeliest outcome in 2015.

However, if the post-Budget shifts in sentiment do persist through the summer, that judgement may change.

See the survey details and full results here

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