George Osborne says protecting taxpayers must be a priority
The Tories have offered to work with the government on laws to enhance financial stability while attacking Bradford & Bingley's nationalisation.
Shadow chancellor George Osborne said he was willing to talk "constructively" with opposite number Alistair Darling on ways to protect banks and taxpayers.
But he said B&B's fate was due to the "clear failure" of the regulatory system established by Gordon Brown.
Despite reservations, the Lib Dems said the B&B rescue may prove a "good deal".
After it was confirmed that the UK's eighth largest bank is being taken into public ownership, Mr Osborne said the Conservatives had proposed a "much better way" of dealing with the crisis.
They support giving the Bank of England enhanced powers to step in early and run troubled banks while trying to negotiate a private sale of assets.
Describing the government's plan for B&B as "elaborate", Mr Osborne said Mr Darling had failed to act in the past year to give the authorities stronger powers to deal with banks in distress.
"We think any government needs to step in to provide financial stability but it should be done in a way which has been clearly thought through in advance," he told the BBC.
He said he was sceptical about guarantees that taxpayers would not have to foot the bill for the second bank nationalisation in a year, saying there was "no taxpayer protection with Northern Rock".
"Our principle is clearly we need to protect the taxpayer. The people who should bear the cost most must be the large institutional investors who took a bet on B&B in the good years and, now the bet has gone sour, must pay the price, not the taxpayers."
Mr Osborne said the Tories would work with Labour, on a cross-party basis, on measures to support the banking system, as in the US.
Mr Darling said he would be "more than delighted" to discuss the way forward with the Tories but said they were "clutching at straws" if they thought B&B could be rescued without state support.
For the Lib Dems, Treasury spokesman Vince Cable said B&B, like many other former building societies, had been brought to a "miserable" end by high-risk lending.
But he said that since the government was acquiring B&B mortgage assets for "virtually nothing", the public may not end up paying the price for this.
"It could eventually turn out to be a good deal for the taxpayer," he told the BBC.
"Depending on the competence with which they [B&B's mortgage assets] are managed, it may prove to be a relatively successful outcome for the taxpayer eventually."