“Within our mandate, the ECB is ready to do whatever it takes to preserve the euro. And believe me, it will be enough.” ECB President Mario Draghi, 26 July 2012.

In 2012, the ECB faced down a mortal threat to the euro: fears of redenomination (the re-introduction of domestic currencies) were feeding a run away from banks in the geographic periphery of the euro area and into German banks. Since Mario Draghi spoke in London that July, the ECB has done things that once seemed unimaginable, helping to support the euro and secure price stability. For example, since March 2015, the ECB has acquired more than €1.3 trillion in bonds issued by euro-area governments, bringing its total assets to just over €3¾ trillion.

So far, it has been enough. But can the ECB really do “whatever it takes”? Ultimately, monetary stability requires political support. Without fiscal cooperation, no central bank can maintain the value of its currency. In a monetary union, stability also requires a modicum of cooperation among governments.

Recent developments in France have revived concerns about redenomination risk and the future of the euro. According to reported statements by senior representatives of the Front National (FN), if their leader (Marine Le Pen) wins the upcoming presidential election, the plan is to exit the euro and reintroduce the French Franc. France would then convert the bulk of its €2.1 trillion in government debt into the new currency, and engineer a depreciation to both make repayment easier and boost the country’s competitiveness.

Representatives of S&P and Moody’s have said that this would be a default—by far the largest sovereign default in history. Given candidate Le Pen is leading in recent polls, and that odds makers give her roughly a one in three chance of becoming President, an FN victory is clearly within the realm of possibility. (Recall that the odds of a Trump victory were no higher a few months before the U.S. elections.)

To understand the consequences—just of the heightened risk, not the realization—consider how other euro-area countries would react if France, indeed, exited the euro. First and foremost, small peripheral economies—Cyprus? Greece? Portugal?—might be inclined to leave quickly. If, as we suspect, Italy and Spain also follow, the euro as we know it would be finished.

Why focus on France and not Italy? In recent months, concerns about Italian banks have been in the news. The Italian banking system is surely weak and in need of recapitalization, as are banks in other parts of Europe. But, in the end, addressing this capital shortfall is a question of how, within the context of the rules of the EU Recovery and Resolution Directive, to come up with the tens of billions of euros needed to keep Italy’s banks afloat. Unless policymakers seriously miscalculate, this is not an existential challenge for the euro.

French redenomination is different. At the core of the euro (and the European Union) is the stable political relationship between France and Germany. A French government that abandons the euro would be a far greater political shock in Europe than Britain exiting the European Union.

As in 2011 and 2012, rising redenomination risk (this time arising from the possibility of a French exit) would boost risk premia and encourage money to flow out of the weaker economies and into Germany. The impact of the first would appear in sovereign bond yield spreads, and the effects of the second in the balances in Target2, the euro-area’s real-time gross settlements system.

The following chart shows the yield spreads between French, Italian and Spanish long-term government bonds and equivalent-maturity German bonds. These yield spreads converged in anticipation of the monetary union that began on January 1, 1999. Between 1999 and 2008, they never rose above 50 basis points. (While not in the figure, the same is true for Greece from the time it joined the euro in 2001 until the crisis intensified seven years later.) Importantly, in recent months, the spreads have begun to widen again. For France, that widening has been 40 basis points. For Spain and Italy, spreads have increased by 30 and 50 basis points, respectively. To be sure, these yield spreads remain far narrower than during the 2011-2012 crisis, but the warning signs are plainly visible.

Long-term yield spreads over German government bonds, month-end, 1990 to February 2017