“We need the American leadership,” he said. “Last year, American leadership helped the Europeans to overcome the economic problems of the austerity and the euro crisis. We need the American leadership on growth, on the fight against protectionism. This is why we hope for a solution, of course, of the internal problems, because we need this leadership.”

Italy’s political landscape has been turbulent for more than a year, with more than eight million voters supporting the new, populist Five Star Movement, led by a former comedian, Beppe Grillo, in the last national elections. Mr. Letta, who once served as an elected member of the European Parliament, said parliamentary elections in May would serve as Europe’s biggest test on whether the momentum toward greater integration and euro zone institution building could be impeded.

“The big risk is to have a European Parliament with 25 percent of Parliament coming from anti-euro or Europe movements,” he said, noting that populist movements in different countries arose from different places on the political spectrum. “It’s very difficult to say left or right. Some of them are racists. Some of them are not racists,” he added. “But they are all anti-euro and all anti-Europe.”

The broad political alienation represented by these parties is partly rooted in that failure of governance, Mr. Letta said, arguing the governments have to become more transparent and responsive. He said legislation in Italy to overhaul public financing of elections would be an initial step to try to show disaffected voters that mainstream parties were trying to respond, as would pending changes to make voting more representative.

He also took encouragement from Chancellor Angela Merkel’s recent re-election in Germany, where she beat back anti-Europe voices for a resounding victory. Asked if Ms. Merkel’s insistence on austerity was to blame for rising populism across Europe, Mr. Letta instead blamed institutional disarray at the European level, saying that it took five years and more than two dozen summit meetings before a political response could be mustered, delays that allowed public fury to grow.

He said the crisis subsided only after the head of the European Central Bank, Mario Draghi, promised to do “whatever it takes” to save the euro, a pledge that eased market pressures on countries like Spain, Greece and Italy but did not resolve Europe’s unfinished institutional challenges. Mr. Letta said that the central bank should not become a “political power” and that Europe must strengthen the political institutions over the euro zone and, as one example, complete and deliver banking integration by year’s end.

“We wasted a lot of money just because of the lack of instruments to avoid financial and banking crises,” he said.