The ratings agency Standard and Poor's gave Paris a jolt on Thursday, announcing "in error" it had downgraded France's creditworthiness. It withdrew the statement, but other signs of trouble are mounting.
The "spread" or gap between French and German 10-year bond yields has never been higher, as investors skip over France and invest in its safer neighbour, and the government's borrowing costs are rising.
France now pays 3.46 percent interest on its bonds, more than twice as much as Germany, although still around half as much as Italy does -- for now.
At stake is France's coveted "AAA" credit rating, any downgrade would be a humiliation for Sarkozy six months before he is due to seek re-election, and a blow for European leaders in their battle to save the euro.
"After Greece and Italy, France?" worried Le Monde's Friday headline, over a stark graphic showing France's €1.7 trillion debt just short of Italy's 1.9 trillion and dwarfing Europe's trillion-euro bail-out fund.
This week Sarkozy scrambled to promise a second round of austerity measures, but Brussels was quick to call them insufficient, markets were unimpressed and some believe the crisis is already here.
"Let's not have any illusions. On the markets French debt has already lost its AAA," said Jacques Attali, advisor to former president Francois Mitterrand and former head of the European Bank for Reconstruction and Development.
"When we see the state's borrowing costs over 10 years and the direction of the Franco-German spread, French debt is treated as AA," he said, judging the government's latest austerity programme "obviously insufficient."
The French government is still officially hoping for one percent growth next year, but the European Commission is more pessimistic, predicting only 0.6 percent, which would make it tough to meet debt reduction targets.
"Concerning 2013, further measures will be needed in order that excessive deficit is corrected," the commission's Economic Affairs chief Olli Rehn warned on Thursday, after studying the French austerity plan.
In the run-up to what promises to be a difficult re-election battle, it is hard to see what Sarkozy might be able to cut -- and his budget minister, Valerie Pecresse -- insists there will be no new austerity package.
The last one, announced on Monday, added €7 billion in cuts or new revenue to the budget, following a €12-billion plan announced in August.
Sarkozy's most dangerous opponent, Socialist candidate Francois Hollande, has accused Sarkozy of adding 500 billion to France's debts during his five-year term with tax giveaways to big business and the rich.
On Friday, out campaigning, Hollande denounced Sarkozy's economic record and warned that "the markets have already anticipated" a French downgrade.
The right, in turn, denounces Hollande's spending plans -- which are limited by the situation but include hiring 60,000 teachers -- as "ruinous".
Polls show the debt and the economy top voters' concerns and, despite a recent small surge in support for Sarkozy, predict a clear Hollande win.
Meanwhile, France's position can not be divorced from that of the wider eurozone, where political and economic crises in Greece and Italy and stalling growth everywhere have called into question the single currency itself.
Unlike Italy, where much of the government's debt is held by domestic investors, France borrows much of its government money from abroad, making it more vulnerable to international market sentiment.
And French banks are holding tens of billions of euros in Italian debt -- any repeat in Rome of a Greek-style partial default would hit the sector hard and strain the ability of Europe's bail-out fund to keep it afloat.
Sarkozy and his ally Chancellor Angela Merkel of Germany have been forced to defend the project once again this week, after reports that officials are quietly drawing up contingency plans for the break-up of the zone.
The French leader was back at the tribune on Friday, warning that the collapse of the European Union could endanger peace on the continent.
"In this global crisis we must find the strength, the energy and the courage to maintain France as one of the great nations of this world and one of the first in Europe," he said.