“The case for full nationalization is far stronger now than it was a few months ago,” said Adam S. Posen, the deputy director of the Peterson Institute for International Economics. “If you don’t own the majority, you don’t get to fire the management, to wipe out the shareholders, to declare that you are just going to take the losses and start over. It’s the mistake the Japanese made in the ’90s.”

“I would guess that sometime in the next few weeks, President Obama and Tim Geithner,” he said, referring to the nominee for Treasury secretary, “will have to come out and say, ‘It’s much worse than we thought,’ and just bite the bullet.”

Image Speaker Nancy Pelosi has alluded to internal debate over whether large banks should be nationalized, while aides to President Obama have avoided the word and are looking into alternatives. Credit... Mark Wilson/Getty Images

So far the Obama administration has signaled that it is trying to avoid that day, and members of its economic team  among them Mr. Geithner and the president’s top economic adviser, Lawrence H. Summers  made the case during the Asian financial crisis in the 1990s that governments make lousy bank managers.

Indeed, the risks of nationalization they warned about then apply equally to the United States now. The first is that nationalization can prove contagious. If the Obama administration took over Bank of America and Citigroup, two of the largest banks in the United States, private investors could decide to flee from the likes of JPMorgan Chase and Wells Fargo, or other major banks, fearing they could be next.

Moreover, Mr. Obama’s advisers say they are acutely aware that if the government is perceived as running the banks, the administration would come under enormous political pressure to halt foreclosures or lend money to ailing projects in cities or states with powerful constituencies, which could imperil the effort to steer the banks away from the cliff.

“The nightmare scenarios are endless,” one of the administration’s senior officials said.

The argument in favor of nationalization, even a brief nationalization of a few months or years, is straightforward: It might be the only way to pull America’s largest financial institutions out of the downward spiral that makes it enormously difficult to raise the capital they need to keep operating.