François Hollande, the Socialist opposition candidate and opinion-poll leader, said that if he is elected France's president in May, he will seek help from the only institution he feels can salvage the euro : the European Central Bank. Speaking in an interview at his campaign's headquarters in central Paris, Mr. Hollande, 57 years old, said only the ECB has enough credibility and financial firepower to restore investor confidence and unravel the debt crisis that has been roiling the euro zone for two years. "Everyone knows that without a powerful intervention by the ECB, it will be impossible to restore calm on markets," Mr. Hollande said. Mr. Hollande's approach would mark a significant shift from the euro zone's latest anticrisis plan, under which members of the monetary union are expected to abide with the ECB's statutory independence. At a recent meeting, French President Nicolas Sarkozy, German Chancellor Angela Merkel and Italian Prime Minister Mario Monti agreed not to interfere with the ECB's policy. Mr. Hollande said he was well aware that Germany would staunchly oppose any attempt to amend the ECB's core mandate : to keep inflation in check. He said that were he to be elected president next year, however, he would still ask that the ECB play a bigger role in stemming the crisis within the framework of current bylaws. "I would rather have a change in practice than a change in the treaty," Mr. Hollande said. "I would rather reach tangible results than get into a stalemate." To assist troubled euro-zone nations, the ECB could commit to buying more sovereign bonds, he said. With Germany's backing, the ECB has so far refused to become a lender of last resort, even as a growing number of euro-zone countries are struggling to raise debt at affordable costs. Last year, euro-zone members agreed to set up a rescue fund to assist the monetary union's most troubled members. But the fund is not yet fully operational and investors fear it won't have the might of the ECB. In the interim, the ECB has accepted to buy bonds issued by the most troubled members. But its president, Mario Draghi, repeated on Thursday that the program was "neither eternal, nor infinite." Mr. Hollande said were he to be elected in May, he would benefit from a "reinforced legitimacy," which would give him a stronger bargaining power to redefine the ECB's role with Ms. Merkel, who faces elections in September 2013. In the first round of the presidential contest, Mr. Hollande would garner 31.5% of the votes, with Mr. Sarkozy at 26%, according to a survey conducted by French polling agency LH2 that was published on Sunday. In the second round, Mr. Hollande would defeat Mr. Sarkozy, getting 57% of the votes. Mr. Hollande, a former Socialist Party chief and head of the Corrèze, a largely depopulated region in central France, has never held a position in government. A graduate of Ecole Nationale d'Administration, France's elite school for civil servants, his political career took off in 1981, when, as an economic adviser to then-President François Mitterrand, he contributed to the implementation of a plan to nationalize the country's biggest companies and banks. The politician, often seen riding a scooter around Paris and known for his sense of humor, has said his goal would be to "pacify" France and turn a page on what he described as Mr. Sarkozy's "divisive" style. Yet, the election campaign is likely to revolve chiefly around the state of France's finances. Mr. Hollande said he was concerned that France's borrowing costs will climb if the country loses its prized triple-A debt rating. He said a downgrade would hit France badly, especially because unlike the U.S.—which lost its triple-A from Standard & Poor's in August—the country wouldn't receive support from the central bank. Talk of a credit downgrade has intensified in recent days after US credit rating agency S&P put France and 14 other euro zone countries on negative watch, saying it was worried about the bloc's continued inability to find a durable fix to the crisis. Mr. Sarkozy spent a large part of this year trying to convince investors his country deserved to keep its triple-A. He passed two rounds of austerity measures this summer, and took steps to enshrine budget discipline in the constitution. In recent days, however, Mr. Sarkozy appeared to have given up hope France could retain its top rating, telling French newspaper Le Monde on Monday that losing it would be "an added difficulty, but not insurmountable." Mr. Hollande said he disagreed. "It will be a political catastrophe for those who govern us, because they presented the triple-A as a national treasure," Mr. Hollande said. "When a team loses the treasure, they don't deserve compliments." Mr. Hollande also criticized Mr. Sarkozy on the recent accord all European Union countries with the exception of the U.K. reached last week to reinforce fiscal discipline within the bloc, saying it was "vague and incomplete." Mr. Hollande said he wouldn't challenge the drive toward deeper fiscal integration and deficit reduction, which he said are "necessary." Yet, there are many legal uncertainties on how EU countries will enforce the agreement, he said, adding that it lacked provisions to boost economic growth. "How can we meet deficit-reduction targets without a return to growth?" he said. In addition to enrolling the ECB, he said, EU leaders should increase collective funding for pan-European projects in areas such as renewable energy and infrastructure. However, he said he agreed with the decision of EU leaders last week to press ahead with the fiscal-pact agreement without the U.K. "We can't accept a situation whereby the U.K. would use the possibility of an agreement to gain advantages for the City," Mr. Hollande said.