There is only one way to save the Euro now. The ECB acts as lender of last resort to the 17 Eurozone governments. But nobody would want to act as lender of last resort to a deadbeat, and the ECB wouldn't want to act as lender of last resort to a fiscal deadbeat. With the guarantee of unlimited loans from the ECB, the fiscal deadbeat would have every incentive to keep on borrowing and spending unlimited amounts. It's a mix of: the free-rider problem (because they are only one in 17, and even less than that for a small country); and the Samaritan's dilemma (if they know you are going to help them get out of trouble, they are not going to stay out of trouble).
The Eurozone lacks a central fiscal authority to match the central monetary authority. And it seems to lack the ability to create a central fiscal authority in the normal way. Nobody seems to have the power to exert that central fiscal authority, and force the 17 governments to do what they are told.
But the ECB does have that power. It can say to each of the 17 governments: "We will act as your lender of last resort if and only if you do what we say. If you don't do what we say, we will loudly announce that we will no longer act as your lender of last resort, and the bond markets will make mincemeat of your bonds, and there will be runs on all your banks."
In fact, the ECB is the only body that does have that power. I'm not talking about legal power. It's long past that stage of the game. Good central banks ignore all the rules in an emergency (as Brad DeLong tells us the Bank of England did for a century). The ECB has the de facto power to save any or all of the 17 countries. But it won't use that power unconditionally. It has to make the 17 governments do what it tells them to do. It has the power to do that. "Do what we say, or your country is toast".
The normal question in political macroeconomy is whether the monetary authority should have independence from the fiscal authority. It's time, in the Eurozone, to reverse that question. Should the 17 fiscal authorities have independence from the one monetary authority?
Is this democratic? Of course not. Might it happen? I don't know.
Update: edeast in comments directs us to this very relevant piece by Jacob Funk Kirkegaard of the Peterson Institute for international Economics.