Suddenly, everything old is New Deal again. Reagan is out; F.D.R. is in. Still, how much guidance does the Roosevelt era really offer for today’s world?

The answer is, a lot. But Barack Obama should learn from F.D.R.’s failures as well as from his achievements: the truth is that the New Deal wasn’t as successful in the short run as it was in the long run. And the reason for F.D.R.’s limited short-run success, which almost undid his whole program, was the fact that his economic policies were too cautious.

About the New Deal’s long-run achievements: the institutions F.D.R. built have proved both durable and essential. Indeed, those institutions remain the bedrock of our nation’s economic stability. Imagine how much worse the financial crisis would be if the New Deal hadn’t insured most bank deposits. Imagine how insecure older Americans would feel right now if Republicans had managed to dismantle Social Security.

Can Mr. Obama achieve something comparable? Rahm Emanuel, Mr. Obama’s new chief of staff, has declared that “you don’t ever want a crisis to go to waste.” Progressives hope that the Obama administration, like the New Deal, will respond to the current economic and financial crisis by creating institutions, especially a universal health care system, that will change the shape of American society for generations to come.