Archibald Primrose (almost equally unknown to most people as the 5th Earl of Roseberry) was one of those glossed over Prime Ministers who fill in the gaps, in this case the transition between the end of the Victorian age and into the early 20th century (a time of a fair amount of political upheaval alongside the switch of centuries). Wittier men than me have commented that his greatest achievement in his ~12 months as PM was having his horses win the derby twice. Damned with faint praise indeed.
He had the misfortune of having to follow Gladstone, when the Grand Old Man finished his last term in Number 10, no easy task to begin with. What made his task all the harder was that there was no leadership election to choose him, nor was Gladstone or leading lights of the Liberal party asked for their opinion.
Put simply, the Queen liked him better than the rest of the possible candidates, both personally and for his policies (he was a Liberal Imperialist against social reform and as foreign secretary opposed to Irish Home rule, both backward steps for the Liberal party at a time when Gladstone's returning reign had already held the party's policies to the lines of a previous age, possibly to the party's long term detriment. See the fast move forward in the early 20th Century when these restraints had been removed).
Now obviously this was all within the rules of the time, the Queen could select someone in she felt could command confidence in Parliament and Roseberry could, albeit not for very long and despite the disapproval of his colleagues who nevertheless weren't going to bring down the government over the appointment. Maybe a better man could have made a go of it. But the nature of his appointment always hung over him and was always a stick to beat him with.
Which brings me to Edward Miliband as I'm told we should call him now. Obviously he has far greater legitimacy in that he was in fact elected leader. But the nature of his victory hangs over him, whether from the close nature of it, the union's influence or the creative campaigning envelopes.
It's not something that in itself is a cause of harm to him, certainly not to the public who (and observe the first rule here) mostly don't know about it, or at best it plays in to a wider narrative of him as a Trade Union man. But it is a stick to beat him with for those who want to undermine him, both within and without the Labour party. If he was doing well and in a strong leadership position I don't believe it would be mentioned much, his legitimacy would be proved by his performances. That it hangs over him (of course a purely personal opinion) is in part evidence of his struggles as leader.
It'd be wrong of me not to mention the Nick Clegg comparison, since in that election (and I voted for him in it) there was a fair number of votes disqualified for being late, which might have swung the contest to Chris Huhne. Of course in the event Huhne took the party view and declared Clegg the winner said he had no problems with the result and importantly stayed part of the team and inside the tent. He wasn't hovering around in the background avoiding any flak and acting as a lightning rod for alternative possibilities or hypotheticals (although in fairness David has now tried to defuse some of that).
But in addition to that I'd argue that Clegg's leadership following his election was generally of a high standard, and also well perceived. By his performances he established legitimacy to the point where the election was a non-issue and is now effectively buried beyond mention. Ancient history.
Edward needs to up his game to bury the election, which he will if he's good enough for long enough, and hope to prove himself rather better than Roseberry (in politics if not horseracing).