FRANKFURT — To euro zone countries in need, euro bonds would be a noble expression of European solidarity and a crucial instrument for preserving the common currency.


To Germans and quite a few others, though, euro bonds would be a lot like co-signing a loan for a deadbeat brother-in-law.


Those caricatures have dominated a debate that has left Europeans deeply divided on a central question: Should euro zone countries create common bonds to reduce borrowing costs for members that cannot get affordable credit on their own?


But despite the intensity of the debate, even as political upheaval in Greece and bad bank loans in Spain mushroom into existential threats to the currency union, the euro bond remains only the vaguest of concepts.


About the only thing clear is that Germany and some other creditworthy northern countries oppose adopting such bonds anytime soon. Meanwhile, François Hollande, the new French president, seems keen on speeding things up — even if he has not quite articulated how his idea would work.


“You don’t know what François Hollande is talking about when he talks about euro bonds,” said Jacques Delpla, a member of the French Council of Economic Analysis, a panel that advises the government. “An open bar with German money for Greece and Spain? That doesn’t work.”


At their meeting in Brussels last week, European Union leaders agreed only that euro bonds deserved further study.


Mr. Delpla is the co-author, along with a German economist, Jakob von Weizsäcker, of one of the few detailed proposals so far. They outlined how euro bonds might be used to ease financial pressure on countries like Greece, Spain or Italy while addressing German concerns by encouraging more prudent government spending.


The basic idea of euro bonds does enjoy wide support among economists. Proponents also include Christine Lagarde, managing director of the International Monetary Fund . And last week the Organization for Economic Cooperation and Development in Paris called for some variation of euro bonds.


The various models share a basic idea: In addition to each country’s raising money by issuing its own bonds, as is now the practice, they would put at least some of the debt into a common pool. These pooled bonds would be issued by some kind of joint European debt agency, with all members assuming shared responsibility for repayment.


There is no agreement yet on whether that pool would be used to replace existing debt or to finance new borrowing. Nor is it clear how countries would have access to the money raised by the sale of the bonds.


Surprising as it may seem, the euro zone’s total government debt actually is lower as a proportion of annual gross domestic product — 87 percent — than that of the U.S. government, which is more than 100 percent of G.D.P.


A reason euro zone debt has reached crisis proportions is that a few countries in Southern Europe owe too much money and have lost the faith of investors.


The United States so far is able to stay a half-step ahead of its debt because it can keep selling Treasury bonds. Investors worldwide are so certain of being repaid that they accept interest rates of less than 1.75 percent for a 10-year Treasury bond.


Right now, though, the weakest euro zone members have no such credibility with creditors. So Spain must pay more than 6 percent on its 10-year bonds, while investors last week were demanding 5.6 percent for Italian bonds. And about the only ones willing to lend to Greece these days are Europe’s bailout institutions and a few roll-the-dice hedge funds.


But if euro zone countries threw their bonds into the same pot, the argument goes, they could create a debt market rivaling that for U.S. Treasury securities — and greatly improve the chances that Spain and Italy could continue to make interest payments and avoid default or a Greek-style debt overhaul.


That is why, as Greece edges toward an exit from the euro zone, and Spain wrestles with its expanding bank crisis, many analysts see euro bonds as unavoidable.


“If we don’t have common bonds very soon, in the next year or so, southern European countries will go bankrupt,” Mr. Delpla said.


The problem is that talk of euro bonds inevitably raises fundamental questions about the nature of the European Union. Such bonds would require European countries to watch one another’s spending much more closely, and each country would have to cede some control over its own budget.


For euro bonds to work the way U.S. Treasury securities do, investors would need assurances that they are backed by a central treasury, or at least an agency with direct access to tax revenue from each member state.