WASHINGTON - President Barack Obama's plan to transform the Federal Reserve into a super-regulator ran into skepticism Thursday from lawmakers who worry that the central bank is not the best suited to keep an eye on firms deemed so big and influential that their demise could hurt the economy.

Senators voiced misgivings as Treasury Secretary Timothy Geithner began a marathon day of selling Obama's financial regulatory plan to give the Fed more authority, create a new consumer protection agency and bring unregulated sectors of the financial markets under government oversight.