Because of the hedging of risks, some were presented as safe, even if the underlying loans were risky. When these securitized assets were being created, federal regulators declined to regulate this rapidly expanding shadow financial system. Alan Greenspan, then the chairman of the Federal Reserve, argued that the complex securities were improving the safety of the banking system by transferring its risks to outside investors.

It turned out that he was wrong, as became clear when the crisis spread last year.

A possible model for the way the new Treasury plan could work arose in a deal last July that had no government involvement. In that case, Merrill Lynch sold $31 billion in securities for 22 cents on the dollar. The buyer, the Lone Star group of private equity funds, put down only one-quarter of the purchase price and had the right to walk away, forfeiting only the down payment, if it later turned out the securities were worth even less than it had agreed to pay.

Thus Lone Star stands to receive the upside profit if the securities prove to be more valuable, but has only a limited downside risk if they do not.

In such a deal under the Obama administration’s plan, it would be the government that stood ready to absorb losses if they were too large, while also providing some of the financing for the purchases.

Any government-assisted deal would probably need much more public disclosure, some economists say, than was made by Merrill and Lone Star, which did not reveal exactly which securities were involved. Presumably, too, the government would want such packages to be shopped widely to get the best price.

“They must disclose fully exactly what the government is buying, or insuring, or providing financing for,” said Simon Johnson, a professor at the Massachusetts Institute of Technology and former chief economist of the International Monetary Fund. “Congress is really hypersensitive to this issue right now. Believing you can get away with the opaque deals we saw in Citigroup or Bank of America would be a misconception.”

In those deals, in which the government either assisted a takeover or tried to shore up an institution’s balance sheet, the government provided insurance against further losses on portfolios of assets, but did not disclose details of the assets.