In Washington last week, Democratic Sen. Sherrod Brown of Ohio previewed his so-called “too-big-to-fail” bank bill, written with Republican David Vitter of Louisiana. “Too big” means banks with more than $500 billion in assets: Bank of America, Citigroup, JPMorgan Chase, Morgan Stanley, Wells Fargo, and Goldman Sachs. Mr. Brown wants them to be able to take care of themselves when they fail.

Under the bill, the megas would be required to retain sufficient assets to withstand a crisis. The bill sets an 8 percent minimum capital requirement for banks with assets of $50 billion or more, and a 15 percent minimum capital requirement for the $500 billion banks.