Just how much could the bailout of the financial system end up costing American taxpayers?

Neil M. Barofsky, the special inspector general for the Troubled Asset Relief Program set up by the Treasury Department, came up with the largest number yet in testimony prepared for delivery Tuesday to a House committee. “The total potential federal government support could reach up to $23.7 trillion,” he stated.

But in the report accompanying his testimony, Mr. Barofsky conceded the number was vastly overblown. It includes estimates of the maximum cost of programs that have already been canceled or that never got under way.

It also assumes that every home mortgage backed by Fannie Mae or Freddie Mac goes into default, and all the homes turn out to be worthless. It assumes that every bank in America fails, with not a single asset worth even a penny. And it assumes that all of the assets held by money market mutual funds, including Treasury bills, turn out to be worthless.

It would also require the Treasury itself to default on securities purchased by the Federal Reserve system.