Isn't it just the worst sort of déjà vu? Like a song stuck at the same false note or a mediocre movie whose dialogue and drama you know by heart, we have seen and heard it all before.

The script doesn't change; nor does the ensemble cast. There's the taciturn terseness of Prakash Karat, the masterly manoeuvring of Pranab Mukherjee and the voluble rhetoric of Lalu Prasad. And they are all spouting the same old lines on "national interest". As always, Sitaram Yechury plays the role of the Congress's favourite Marxist; the allies scurry about saying one thing in public and quite another in private and the Congress President says very little to anyone — aware perhaps, that the precarious space between 'Yes' and 'No' is ultimately hers and only hers, to navigate.

And then there's the Prime Minister. Over the past two years it is clear to anyone who knows him that he sees the nuclear deal as the centrepiece of his political legacy. The Left may make acerbic remarks about how this isn't a deal between George Bush and Manmohan Singh, but one between two countries. In other words, the Marxists argue that if needed, the deal can be sealed by the next government as well and the PM has been blinded by the need to leave his personal imprint on history. It's a pretty unfair criticism. Manmohan Singh has been called many things by those who don't like him — "weak", "ineffective" and "oversensitive" among them — but no one has ever called him egotistical. Anyone who has spoken to him on the nuclear deal knows that he passionately believes that nuclear energy can transform India's future, as will the end of nuclear apartheid.

But now the Prime Minister has to confront his own moment of truth. If he believes — as he does — that the nuclear deal could redefine India's sense of self — what he does or doesn't do next will determine his own claims to assertive leadership.

For months now, the PM has alternated between being cajoling and aggressive on the nuclear deal. So, at times, he erupts in frustrated anger and suddenly tells The Telegraph that the Left can walk out of the coalition if they wish; he knows what he needs to do. At other moments, stonewalled by the pressures of realpolitik, he has retreated into making mild proclamations on coalition dharma.

Every time the Left and the UPA stand eyeball to eyeball and put troops in position for the next round of battle, rumours surface that the PM has told his party he will resign if India reneges on the nuclear deal.

And if he wants his words to have any weight and not be dismissed as momentary emotional despair, it may be time for Manmohan Singh to show that he means what he says. In days from now when the Left parties tell his government that its time is marked if it walks the next step on the nuclear journey, the PM must show India that he has the courage of his convictions.

And if his party, which has dithered and dragged its feet on this single issue for an indefensibly long time, aborts the attempt, the PM must quit. Or else, he will be remembered as the man who chose to trade his proudest political decision for seven extra months in South Block.