The politics of populist anger are on the march across Europe, fueled by austerity, recession and the inability of mainstream politicians to revive growth.

In Greece, the main problem is Golden Dawn, whose violent neo-Nazi thugs have finally provoked a belated government crackdown. In France, it is the anti-immigrant, anti-Europe, National Front, whose clear victory in a local council race on Sunday sent shock waves through both mainstream parties. And in Italy, it is the anti-politics Five Star Movement, which feeds on popular resentment of an out-of-touch political class, austerity directives from Berlin and Brussels, and increased migration of refugees from North Africa and the Middle East.

Prime Minister Enrico Letta of Italy, who is scheduled to meet with President Obama in Washington on Thursday, warned this week of the danger that rising populism poses to political and economic stability across Europe, especially given the likelihood of strong populist gains in next spring’s European Parliament elections. But it will take more than warnings to turn back this tide. Mainstream governments like Mr. Letta’s must convince voters that they can do more than administer the austerity demanded by Germany and can deliver the economic relief demanded by their citizens.

Mr. Letta, in office since April, has, until now, been constrained by his need to avoid offending former Prime Minister Silvio Berlusconi, who controlled enough votes to bring down the government. But after winning a parliamentary showdown with Mr. Berlusconi this month, Mr. Letta is freer to act on needed reforms.