Back in 2010, Senator Sherrod Brown, Democrat of Ohio, and Mr. Kaufman were co-sponsors of the Safe Banking Act, which proposed placing tough limits on banks’ size. If it had passed, it would have imposed a strict 10 percent cap on any bank holding company’s share of United States deposits and set a 6 percent limit on leverage.

“We were disappointed,” Mr. Brown said in an interview on Thursday. “Not only did Treasury oppose it, but they proudly opposed it. If the Treasury had spoken out for it we could have gotten very close to winning.”

Thankfully, Mr. Brown has not given up on the idea of reducing big banks’ size and threat to taxpayers. He and Senator David Vitter, a Louisiana Republican, have asked the Government Accountability Office to quantify the size of the advantages — and implied taxpayer subsidies — that large financial institutions enjoy over their smaller brethren. The study is expected to take about a year to complete.