The most immediate expense will come in the next several weeks, when federal bank regulators complete “stress tests” on the nation’s 19 biggest banks. The tests are expected to show that at least several major institutions, probably including Bank of America, need to increase their capital cushions by billions of dollars each.

The change to common stock would not require the government to contribute any additional cash, but it could increase the capital of big banks by more than $100 billion.

The White House chief of staff, Rahm Emanuel, alluded to the strategy on Sunday in an interview on the ABC program “This Week.” Mr. Emanuel asserted that the government had enough money to shore up the 19 banks without asking for more.

“We believe we have those resources available in the government as the final backstop to make sure that the 19 are financially viable and effective,” Mr. Emanuel said. “If they need capital, we have that capacity.”

If that calculation is correct, Mr. Obama would gain important political maneuvering room because Democratic leaders in Congress have warned that they cannot possibly muster enough votes any time soon in support of spending more money to bail out some of the same financial institutions whose aggressive lending precipitated the financial crisis.

The administration said in January that it would alter its arrangement with Citigroup by converting up to $25 billion of preferred stock, which is like a loan, to common stock, which represents equity.

After the conversion, the Treasury would end up with about 36 percent of Citigroup’s common shares, which come with full voting rights. That would make the government Citigroup’s biggest shareholder, effectively nudging the government one step closer to nationalizing a major bank.