As Tony Blair marched inexorably on towards his 1997 landslide victory, I and fellow members of the Daily Telegraph's leader writing team clung to a pathetic last hope, like the bedraggled remnants of a Native American tribe holding fast to its ghost dance. No British political leader with less hair than his opponent, we solemnly reassured each other, had won a recent general election. Remember Thatcher and Kinnock! Wilson and Home! Macmillan and Gaitskell! John Major, with his mop of grey hair, would win out over that incipient baldy, Blair.
This desparate last hope turned out to be a pile of old horsefeathers (and isn't soundly based, in any event: Edward Heath had more hair than Harold Wilson in 1974, and lost to him twice). I mention this to give some context to the claim I mention next. IPSOS-Mori has been asking the question "Who is the most capable Prime Minister?" for a very long time. No party leader since Thatcher has emerged as Prime Minister after an election in which her opponent was judged as most capable.
Thatcher led Michael Foot by 46 points to 15 just before the 1983 election, and Kinnock by 45 points to 27 just before the 1987 one. Similarly, Blair led William Hague by an even more emphatic 51 points to 15 just before the 2001 poll. In May 2005, he led Michael Howard by 40 points to 21. Major's led Kinnock in 1992 by 38 points to 27. Furthermore, party loyalty was far stronger in the late 1970s than it is now – so that Thatcher ran behind Callaghan is less important than Miliband running well behind Cameron.
In 2011, Cameron led Miliband by 45 points to 25, which is a Blair-over-Howard type margin. Ah, you may say: so the figure is out of date. Not really: another poll of voters by the same company found that two-thirds of them believe he's not up to being Prime Minister. As Anthony Wells of YouGov pointed out in an immortal post, precendents are there to be broken (also see here) so Miliband may beat this one. But to be so far behind when compared head-to-head with Cameron doesn't bode well for him.