There would also be political questions within the core. Many West Germans were appalled by the cost of national unification when East Germany merged into the current German state. If they did not like subsidizing people who in some cases were their cousins, how will they like subsidizing Greeks or Portuguese?

Perhaps of more importance, how can the troubled economies regain competitiveness within Europe? Before the use of the euro, when Europe tried to maintain exchange rate stability but kept separate currencies, there were periodic sharp devaluations of some currencies, most notably the Italian lira. That made those countries competitive again, for a time.

Europe’s Growth and Stability Pact, which sets the rules for euro membership, limits the size of budget deficits. It was supposed to prevent such problems. But it was largely toothless, especially after Germany and France found it convenient to violate its terms when it suited their economic needs.

Image At a market in Lisbon. Portugal scaled back its short-term borrowing, roiling world markets. Credit... Mario Proenca/Bloomberg News

Political union would not cure the underlying economic problems, but it would make it easier for a European government to provide assistance to ailing areas through transfers of tax revenue and special spending programs, and for Continentwide laws to be enacted even if they were deeply unpopular in some areas.

Even then, problems could arise. In the United States now, some states, including California, are in severe financial straits. California represents a larger part of the American economy than Greece does of the European one, but even if it did default it would not create a national debt crisis, and Washington could provide help.

The world has sought currency stability across national borders many times. The gold system was such an effort, but it retained separate national currencies whose value against gold could be adjusted if need be. Some economists think that maintaining artificially high exchange rates was a cause of the Great Depression.