Past performance is no guarantee of future success, whether the caveat is applied to the value of a share portfolio or the record of the opposition in holding government to account. Political stock can dip as well as rise. Unusually, it was David Cameron's Conservatives who had a bad day yesterday. This was unexpected for two reasons. First, the Tories had enjoyed the upper hand in the Commons recently, scoring notable hits against Gordon Brown's government. Secondly, the nationalisation of Northern Rock presented Mr Cameron and George Osborne, his shadow Chancellor, with the opportunity to land damaging blows on Labour's reputation for economic competence.

Or so it had seemed until Mr Cameron and Mr Osborne embarked on a strategy that can be described, at best, as questionable. The Tory leader called for Alistair Darling, Mr Brown's Chancellor, to be sacked. Mr Osborne described Mr Darling as, politically, a dead man walking. The attack was twin-pronged: to demonstrate that nationalisation was not business as usual for Northern Rock employees, savers and mortgage holders (and, therefore, could not reassure them or be sustainable in delivering stability); and to undermine the credibility of the Prime Minister and his Chancellor by linking them directly with the failed nationalisations of the Labour governments of the 1970s.

So far, so good. But what would the Tories have done in Labour's stead? It does not require the mind of a Machiavelli to know that mud does not stick unless the thrower has a credible alternative to the course of action targeted for demolition. Here the Tories were found wanting, and have been since the Northern Rock crisis began. They have invested too much in the destructive side of opposition.

Parties tend to be rumbled when they behave like this. Mr Osborne, in particular, left the door open for Mr Darling to accuse him of naked opportunism, implying that this was more important to the Tories than safeguarding the taxpayer's interests. With a stake of perhaps £3500 each in Northern Rock, this is a substantial interest to represent and protect. So, what would the Tories have done? As far as can be ascertained, take Northern Rock into public administration under the Bank of England until a suitable buyer could be found. As Vince Cable, the Liberal Democrat Treasury spokesman, pointed out, this was nationalisation in all but name.

Would the Tories have accepted a bid from a private suitor under which the taxpayer would have continued to shoulder the risks and liabilities while the new owner would reap the benefits? Doing so would have ill served the taxpayer's interests. That is why Mr Brown says the option was rejected. Nationalisation is far from ideal and questions remain about the government's handling of the unfolding crisis. It has taken the least bad option; better this than the financial turbulence from allowing the bank to go under. Better this, too, than selling it off cheaply in a deal that would have left the taxpayer with a sizeable, long-term liability.

Credibility is a make-or-break commodity in politics. Oppositions, to be effective, must demonstrate it, too. Mr Cameron and his shadow Chancellor have been found wanting on that score. Holding the government to account is about much more than cheap political point-scoring.