What the Chancellor has in mind - and what may be announced in Wednesday's pre-budget report - is not a classic windfall tax, which would be levied on banks' profits.
Instead it would be a super-tax on bonuses over a certain level paid to British based investment bankers.
I am not sure how brilliantly clever it is for senior Conservatives to brief that they will be supporting it only for political reasons, while believing that the policy they are backing "might be bad for the UK".
George Osborne, the shadow chancellor, did not pre-empt the government by announcing that a Tory administration would impose such a super tax - although he did consider announcing an intention to impose a one-off special tax on both big bonuses and dividends.
That said if the government announces such a super tax, the Tories won't oppose it.
As one senior Conservative told me, such a super tax might be bad for the UK, but it would not be great for the Tories to oppose a tax that may give a nice warm feeling even to many of their core supporters.
Well, if Her Majesty's Opposition are going to be backing Brown and Darling, can there be any doubt at all about whether the Tory grassroots and blogosphere would be full square behind them?!