Listen to a top economist in the Obama administration describe Paul A. Volcker, the former Federal Reserve chairman who endorsed Mr. Obama early in his election campaign and who stood by his side during the financial crisis.

“The guy’s a giant, he’s a genius, he is a great human being,” said Austan D. Goolsbee, counselor to Mr. Obama since their Chicago days. “Whenever he has advice, the administration is very interested.”

Well, not lately. The aging Mr. Volcker (he is 82) has some advice, deeply felt. He has been offering it in speeches and Congressional testimony, and repeating it to those around the president, most of them young enough to be his children.

He wants the nation’s banks to be prohibited from owning and trading risky securities, the very practice that got the biggest ones into deep trouble in 2008. And the administration is saying no, it will not separate commercial banking from investment operations.