Partly this was because the hard painstaking slog in my local marginal was clearly beginning to pay off for the MP and his battalion of foot-sloggers. Much as one prefers wars of maneouvre over wars of attrition, the bold decisive stroke over the hard grind - sometimes only the long slog will do the trick. Hey diddle diddle, straight up the middle, as the US Marines are wont to say.
Partly it was what I could see of the Crosby-Osborne strategy at work, which was clearly achieving the modern goal of getting inside the other guys' decision-loop (the Northern Powerhouse initiative being a case in point).

But it was also built on a foundation of confidence that Miliband was a strategic loser - which goes beyond his splendidly 'un-Prime-Ministerial' demeanour (Kinnock syndrome) and his proclivity for periodic gaffes. I shall explain.
From his election as leader right up until election day, Mili was widely reported to exude a calm, other-worldy certainty of 2015 triumph. UK electoral history was always broadly against him, as everyone knew: the economy was more-or-less bound to recover over a five-year period; and few losing parties have ever turned things around in one cycle. And yet he was calm and certain.
This is crazy, lazy stuff, at once messianic and (whenever any modern politico-atheist reckons history is on his side) Marxist - and we know where he got that from. The practical outworking of this heartwarming optimistic fatalism in terms of his electoral thinking was the famous '35 per cent' strategy, on which Mr Google will furnish you a heap of references, many of them derisive, many of those from the Labour camp itself.
Now of course it might indeed have been possible (just about) to get a Labour majority with 35% of the popular vote: so far, so good, but that's not the point. I tend to set these things in the military idiom, the relevant slogan being: if you want to hold a river, you must hold both banks. In any walk of life, if you want to stand still, you must advance. In Mili's case: if you want to scrape home with 35%, you must be targetting 40. If you target 35 (or, still worse, if you think 35 is yours for the taking), you will get ... the fruits of your laziness.

As the months passed I became ever more convinced - from both media reports and his public demeanour - that he was possessed of a belief in historical inevitability; and that this would fatally undermine his commitment to driving forward all the necessary hard thinking and hard work. But since there was nothing historically compelling about this particular belief*, and additionally because God helps those who help themselves (which was the Tories how were operating, at both local and strategic levels), I became increasingly sanguine about the whole affair.
ND
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* In truth, during the period May 2010 - May 2015 the only person entitled to believe in 'inevitable' was George Osborne, as regards the economy more-or-less righting itself in that timeframe.