PMQS verdict: Harman wrong-foots Cameron with declaration of war on Heathrow

One of the benefits of a change of chief executive at a corporation is that you get twelve months or so of honesty. The new guy has a built-in incentive to publicise everything which has gone wrong so he can pin the blame on the old guy. This tactic doesn't work so well in politics because political parties suggest a degree of continuity regardless of the change of personnel. You can blame the old guy if he was from another party, but it's harder if you both have the same membership card in your wallet.

There are exceptions however and today was one of them. Ed Miliband had threatened to leave Cabinet over Heathrow. He would not budge on it in opposition. It would have been impossible for him to switch now to support it. Harriet Harman has none of those problems. With certain key pre-publicised conditions, the Labour party is going to support Heathrow expansion, barring a significant change of heart from a new leader.

Today's PMQs was a declaration of war. Harman put Cameron on notice that Labour is going to make this as difficult as possible. She can take advantage where Miliband could not. Cameron will be pitted against up to half his own Cabinet, many of his MPs, his most likely leadership contender in the form of Boris Johnson and his most likely choice for London mayoral candidate in the form of Zac Goldsmith. He will also be pitted against his own strongly-worded opposition to the idea from days gone by. And Labour will be there throughout, making it all as public and painful as possible.

The prime minister tried frantically to escape by laying out the two reasons why he could in no way discuss the issue. Firstly, the report had only just come out and was very detailed, so time was needed to study it. Secondly, commenting now ahead of reading it would make any decision they later took vulnerable to judicial review (probably true). But he promised to come to a decision, one way or another, by the end of the year.

Harman played with her prey. The prime minister was being "bullied by Boris", she said. He was in a "holding pattern above Heathrow and Boris won't let him land".

Cameron got so he actively switched to the subject of his own failure to abide by child poverty targets. "It's an interesting day when a Tory prime minister wants to discuss child poverty and the leader of opposition wants to talk about an airport report none of us have read yet," he spluttered. He was right. It was an interesting day. And the fact he has scrambled around for his own failure on child poverty to escape Harman's questions showed how potentially damaging it is for him.

The Speaker mischievously, and with a characteristic sense of theatre, kept Goldsmith until the end. The Richmond MP and prospective mayoral candidate did a strange little dance with Cameron.  The Howard Commission had spent £20 million writing a report to substantiate the view they already had going into it, he told him. Would the prime minister "engage in real arguments in a way the Howard Commission has not?"

Here at least was a way for Cameron to avoid an immediate confrontation. "The promise I can give him is that this very thorough report is going to be properly studied," the prime minister replied. It was as good an entente as they were going to get. But it won't hold for long. Labour is poised to make the next five months as damaging as possible for him.