In the Sunday Times, the YouGov president (and former NS political editor), Peter Kellner, one of the most respected pollsters and political analysts around, examines why Cleggmania and the Lib Dem surge is "doubly bad news" for the Tories (and not Labour):
First, the Tories were banking on winning 10-20 seats from the Lib Dems to help them win power. That prospect now looks unlikely; indeed, the Lib Dems might start taking seats from the Tories.
The second factor could prove more important. The Lib Dem surge is hurting Tory prospects in Labour-Conservative marginals. Individual nationwide polls cannot detect this; but because YouGov is questioning different samples of about 1,500 people daily, we can combine a week's polls into a total sample of more than 10,000 and see what is happening in different kinds of seat.
YouGov has compared the results from BC (before Clegg) and AD (after debate) in the 115 Labour-Conservative marginals that would fall to the Tories on a swing of 8%. In 2005 Labour's overall lead in these seats was 9%.
In this election, during the BC days, the Tory lead in these seats was 4%. Compared with 2005, that represented a swing to the Tories of 6.5% in these target seats, compared with a swing of 5% nationally.
In other words, the Tories were doing better where they needed to win than in the rest of Britain, and stood to capture 94 Labour seats. Add in, say, 10 gains from the Lib Dems and the Tories would be just 12 short of an overall majority.
The AD pattern is different. Our sample of 2,220 in these target seats now puts Labour one point ahead. The swing since 2005 is down to 4% in the Labour marginals -- the same as the national swing. Not only is the prospect of big Conservative gains from the Lib Dems slipping away; the bonus swing the Tories had been enjoying in the Labour marginals has also disappeared.
The Lib Dem surge has hurt the Tories with special force in Labour-Conservative marginals. The 10-point gain in Lib Dem support in these seats has been overwhelmingly at the Tories' expense.
On these figures, the Tories would gain only 57 Labour seats. And if they prove unable to supplement these with gains from the Lib Dems, the Tories would not even be the largest party.
If the Tories don't end up as the "largest party" on 7 May, it will be as a complete and utter disaster, as I noted in this blog post.
On a side note, News International bosses -- in between barging into the Indie's offices -- must be weeping over how much money they've forked out to YouGov for daily polls that produce ever narrower leads for the Tories and damning analyses from Kellner.