WASHINGTON — It's the largest government bailout in U.S. history and two days after it was introduced to the Americans paying for it, the proposal is still largely a mystery.

Among the unanswered questions: How will the government mop up the bad mortgage debt on banks' books, who will run the process and how much will it cost?

Key elements of the plan remain in flux as behind closed doors Democrats demand modifications that would provide more help for ordinary Americans in return for bailing out the country's financial giants.

In the three-page draft legislation the Treasury provided lawmakers on Saturday, the administration's plan seemed straight forward.

The Treasury asked for $700 billion, which it proposes to spend over the next two years to purchase what Treasury calls "troubled assets" from financial institutions including banks, thrifts, credit unions, broker-dealers and insurance companies.

The assets are defined as residential and commercial mortgages and any securities based on those mortgages if the debt was issued before last Thursday.

The Treasury's first draft said that only mortgage-related assets would be purchased. But in a later version the Treasury secretary asked for the power, after consulting with the chairman of the Federal Reserve, to expand purchases to troubled assets beyond real estate if both officials determine such purchases are necessary to promote market stability.

That would conceivably leave taxpayers picking up the tab on things like bad car loans and credit card debt. The Treasury will likely focus virtually all of its attention on the mountain of bad mortgage debt, however, since that is at the heart of the 14-month long credit crisis.

How would the purchase of this bad debt work? Again, the Treasury draft legislation leaves a lot to the imagination. But Treasury officials have talked about employing a "reverse auction." Under the process, the Treasury would advertise an auction, seeking to buy, for example, $1 billion of subprime mortgage loans that originated around the same time.

In a reverse auction, the financial institution burdened with the bad loans agrees to take the lowest amount bid for the package. A bid of 50 cents on the dollar for a bundle of bad loans would beat out someone only willing to take 60 cents on the dollar.

The banks get to unload their bad debt and the government holds the asset either until it reaches maturity or until the market improves enough for the asset to be sold, perhaps for a profit.

This auction process holds one big advantage, economists say, while also posing a major risk for some financial institutions.

By purchasing the debt, the government would be creating a market that makes pricing easier and more uniform among institutions. That could clear up a huge amount of uncertainty in the market for subprime mortgages. But the clarity could bring bad news to some institutions: The writedowns they have taken so far could leave them with inflated prices for the bad debt remaining on their books.