The Obama administration’s plan to increase the powers of the Federal Reserve, says one critic, is like giving a teenager “a bigger, faster car right after he crashed the family station wagon.” Treasury Secretary Timothy Geithner disagrees. He argues that the Fed is “best positioned” to oversee key financial companies, and that the Obama plan would give the Fed only “modest additional authority.”

Mr. Geithner is right about one thing: The Fed’s power is already vast.But it wasn’t even well-positioned to supervise the likes...