"Let Bartlet Be Bartlet". The defining line, from the defining episode of the defining political series. The moment The West Wing's fictional president casts off the shackles of expediency and decides to become a focus group of one.

Ed Miliband's inner circle are fond of that line. "He's going to say what he thinks," one adviser told me just after his election. "He's not going to pick positions just because they're popular, or because they help his image. Ed's going to be Ed."

There's just one problem with this clamour for Ed to be Ed. It's that on Saturday 25 September 2010, Ed Miliband ceased to be Ed Miliband, and became leader of the Labour party.

Sincerity in politics is good. As is honesty. The public loves unspun politicians. We know because they tell us so.

We also know what happens when they encounter one. A couple of weeks ago, Eric Joyce wrote an article condemning the hypocrisy of an electorate who simultaneously demand authenticity and purity from their elected representatives. Was he praised for his candour? Was he hell. He was crucified in the press and forced to issue a grovelling apology.

On last week's Today programme, Miliband ran head first into this political reality. One described it to me as the "Palin interview", comparing it with the Republican vice-presidential candidate's car-crash encounter with CBS journalist Katie Couric during the 2008 campaign.

For those who didn't hear it, Ed was robustly Ed. He'd toyed with nipping out for a chat with the student protesters. He would fight to the death for the squeezed middle – whoever they are. He was committed to retaining the 50p tax rate. Out of principle, he said proudly, not necessity.

As Ed Balls helpfully pointed out, the Labour movement is crying out for leadership. But there is leading and there is extemporising. And a stream of Zen Labour consciousness is no substitute for a political strategy.

There are many Labour MPs who share Miliband's empathy with the student protesters. But they understand the spectacle of the Labour leader being kettled by the Met may not be the best advert for their party. A similar number share his instinctive desire for progressive taxation. But again, they realise allowing the Tories to paint their party as tax-hike fetishists is politically suicidal.

Miliband will not succeed through deception. He cannot pretend to be something he is not. But self-discipline and pragmatism should not, indeed cannot, be alien concepts to someone who aspires to the highest political office.

Labour's new leader knows this. He won precisely because of his brilliant redefinition from Brownite policy geek to champion of a new political generation. But the keen political antenna that steered him past his brother must also make him aware his intellectual liberal instincts, while creditable, do not automatically place him on a trajectory to power.

Aaron Sorkin is a great screenwriter. But the challenges facing Labour are no work of fiction. It's time for the real Ed Miliband to sit down. And the prime minister within him to stand up.