Ed Miliband could be forgiven at times for wondering why he wanted to be Labour leader.

It has been a bruising week, with mutterings of a leadership plot against him amid anaemic poll ratings. A 2015 election victory that seemed likely a year ago appears now to be far from a dead cert.

It would of course be premature to predict a Labour defeat. There are still nearly six months to go until the ballot and yesterday Mr Miliband proclaimed his determination to win, insisting he would put up with whatever was thrown at him.

Senior party figures, meanwhile, hit back at the negative press by suggesting there had been a campaign against Mr Miliband orchestrated by the party's opponents.

Whether orchestrated or not, Mr Miliband's image and appearance is now under scrutiny to a punishing extent. It is not surprising his supporters regard such stories as silly distractions - how a man looks as he eats a bacon sandwich or how much change he has in his pocket to give a homeless person is obviously no measure of what kind of a Prime Minister he would make.

But, like it or not, in an image-driven age the charisma of politicians has never been more important. In spite of championing popular policies such as the energy price freeze and raising the minimum wage to £8, Labour now has a problem. The latest poll on Westminster voting intentions showed the party trailing the Conservatives by three percentage points, while Mr Miliband's personal ratings have struggled since the summer, with fewer now regarding him as Prime Ministerial material; David Cameron, benefiting from nearly five years in the job, is faring better.

And while there is no sign of a leadership challenge so far, there have been expressions of discontent by Labour backbenchers about their leader's performance and the chances of him taking them to victory in May.

Changing the leader now could fatally undermine the party's election hopes. And who would take over? Shadow Home Secretary Yvette Cooper has been tipped, but lacks popular appeal herself; former Cabinet minister Alan Johnson, who has both charisma and political stature, does not want the job. The party must unite behind Mr Miliband; his attack yesterday on Ukip for its record on the NHS, gay marriage and single mothers was clearly a reminder to supporters that in him, they have a leader who will champion core Labour values.

Still, with victory next May far from assured, some party insiders may look back on how Mr Miliband was elected. It was union backing under the old electoral college system that famously allowed him to pip his brother to the job in 2010. Since then, Mr Miliband has backed reforms of the election system based on the principle of one member one vote (though not straightforwardly so, since they give an equal vote to members, affiliated supporters from trade unions, and registered supporters). It remains to be seen whether the new system will be better at producing voter-friendly leaders, but some in Labour might be reflecting that the old system did not always lead to ideal choices.