Nick Clegg trounced Harriet Harman. The Deputy Prime Minister triumphed by expressing a scorn for Labour which sounded entirely genuine, and also slightly unexpected.

This was not the performance of a man who was hedging his bets, because he thinks that in 18 months’ time he might be forming a coalition with Labour. Clegg, standing in for David Cameron, instead launched a splendidly uninhibited attack on the Opposition for being so vacuous and refusing to give any account of what its economic policy would be.

One expects Cameron to take that line. For Clegg to do so was more damning. He accused Labour of “a bankruptcy of ideas”, of being “economically illiterate”, and of indulging “in pure fantasy” when it calls for “a fantasy freeze” on energy prices.

Here is a man fighting for his political life. If the opinion polls are any guide, the Liberal Democrats are on course for losses in 2014 and 2015 which could well end Clegg’s career.

There was an admirable impenitence about his performance. He allowed no twinge of regret to enter his voice, but instead declared that “without the Liberal Democrats there wouldn’t be a recovery”.

How Labour roared at this provocation. Ronnie Campbell, the former miner who has sat since 1987 for Blyth Valley, rocked back and forwards with incredulous laughter, smacking his knees with his hands and looking as if he might die of apoplexy.

But would there be a recovery without the Lib Dems? This was the question left hanging in the air. Without a coalition between the Conservatives and the Lib Dems that has brought some stability to the public finances, would the recovery have progressed even this far?

That is not a comfortable question for Labour. For Clegg, to claim a share of the credit for the recovery is a political necessity, and he seized his chance to do so with shameless courage.

Harman, who was standing in for  Ed Miliband, is a courageous politician, who saw off five other contenders to become Labour’s deputy leader. Week by week she has to sit listening to Miliband at PMQs, and her demeanour often suggests she is making a loyal attempt to conceal how little she thinks of him.

This week, she failed to show him up by doing any better herself. Part of her problem was a severe lack of ammunition. She began on energy prices, a well-worn theme to which she was unable to bring anything new. And this could not be a prelude to a wider attack on the coalition’s economic policy, because Labour has not worked out what to say about that, except to complain about the undoubted squeeze on living standards.

Clegg might have been caught out by a splendidly double-edged question from one of his predecessors as Lib Dem leader, Charles Kennedy, who said that on our relations with Europe, “actions actually speak louder than words”, and the record of Cameron and his Tory colleagues is actually a pro-European one of which Clegg and other Lib Dems can be proud.

The Deputy Prime Minister acclaimed Kennedy’s “mischievous wit and wisdom”, but did not allow himself to be caught out by these qualities. Peter Bone, a backbencher who can be relied upon to express reactionary views, suggested that Clegg is “turning into a Tory”.

But even this suggestion did no harm to Clegg. For what he had actually demonstrated is that he still believes in the coalition, and is delighted to have the chance to defend its record in government. Labour meanwhile has not demonstrated what it believes in, except for getting back in to power for power’s sake.