French Socialist Party candidate for the 2012 presidential elections Francois Hollande delivers a speech during a campaign rally in Le Mans, western France on Thursday. (David Vincent/AP)

Francois Hollande, a second-fiddle fixture for years in French politics, has over the past 10 months emerged as a possible giant-killer.

According to polls, the roly-poly glad-hander with a ready wit and perpetual smile has a real chance to block Nicolas Sarkozy’s reelection and become France’s first Socialist president since 1995.

With Europe in the grip of a punishing debt crisis, Hollande and his Socialists would have little margin for abrupt change should he replace Sarkozy’s conservative government in the two-round vote April 27 and May 6. But a Sarkozy defeat after only one five-year term would mark an unusual reversal for a politician who promised “rupture” in the way France does business and, with relentless energy, has propelled himself to prominence on the European stage.

The opening for Hollande’s unexpected challenge came in the disgrace of Dominique Strauss-Kahn, the International Monetary Fund chief who was considered an easy winner for the Socialist Party nomination until he was accused in May of sexually assaulting a cleaning woman at a New York hotel. The charges were dropped when prosecutors judged the woman unreliable. But other sex-related accusations against Strauss-Kahn have since arisen in France, and the onetime luminary has vanished from the political stage.

As a political rally here last week demonstrated, however, Hollande has developed his own following in months of tireless campaigning, during which he has pledged to soften the impact of the debt crisis on France’s poor and unemployed and has attacked Sarkozy as a heartless friend of the rich, accusing him of failing to carry out his 2007 promise to allow French workers to “work more and earn more.”

“The president of unkept promises cannot now be the candidate of new proposals,” Hollande shouted, his voice hoarse from a day of speechmaking, street walkabouts and a visit to a dairy farm.

France cannot reduce its debt by cutting into unemployment benefits and health-care budgets, he declared, but must instead impose higher taxes on capital gains and close loopholes that allow the rich to pay smaller proportions of their incomes than wage-earners. In addition, he said to raucous applause, the government must step in to cap the compensation and retirement packages of senior executives, which he noted seem to rise annually despite the financial crisis.

“Francois, president, Francois, president,” chanted about 1,300 banner-waving supporters who jammed the convention center in this western city celebrated for its 24-hour automobile race.

When Hollande’s hour-long speech wound to a close and the crowd intoned the French national anthem, “La Marseillaise,” the candidate raised his arms in salute and moved to the front of the stage to bask in what appeared to be perfect contentment over the noisy display of support.

Rising from obscurity

At almost the same time, Sarkozy was in the northern city of Lille, making a loudly applauded campaign appearance of his own. Hollande, he charged, is lying to the French people, making promises that cannot be kept while France struggles to lower its perennial deficits and using sleight of hand to conceal the real numbers for what he proposes.

Sarkozy, 57, has said repeatedly that he should be reelected because France needs a steady and experienced hand at the helm while it weathers the Europe-wide debt storm. But opinion polls for months have put Hollande in the lead, not so much because voters like him, the pollsters say, but because they dislike Sarkozy’s often aggressive attitude — and because Sarkozy was in charge when the crisis hit.

Sarkozy’s lieutenants insist that the president, who formally announced his candidacy only two weeks ago, will narrow the gap now that he is campaigning full time, armed with the prestige and logistical means attached to France’s near-regal presidency. The latest survey showed he has closed to 27 percent against Hollande’s 28 percent in the first round but remained a distant 44 percent to Holland’s 56 percent in the runoff.

Regardless of whether he wins, Hollande, also 57, has dramatically changed the way he is perceived by Socialist Party supporters and his adversaries alike.

For years, the bespectacled graduate of France’s best schools toiled in the shadows of Socialist figures. He was an obscure assistant to President Francois Mitterrand in 1981, later a deputy to Mitterrand’s spokesman, then an aide to Prime Minister Lionel Jospin. Finally, Hollande was named to head the party, a thankless task but one that gave him a taste of power.

Since they were classmates at the National Administration School, the hothouse for French leaders, Hollande had been in a relationship with Segolene Royal, with whom he has four children. Both partners yearned to run for president in 2007, but Hollande stood aside when he saw that the polls and hard-line Socialists favored Royal.

At the same time, the personal partnership was breaking up. Hollande had taken up with a television presenter named Valerie Trierweiler. In a campaign book published Thursday, Hollande wrote that he felt enormous sadness at seeing Royal go down in defeat at the hands of Sarkozy, not only because of the political loss but also because of the simultaneous personal loss.

Longtime image problem

In a recent political biography by Marie-Eve Malouines of France Info radio, “The Nice Guy’s Strength,” Hollande was depicted as far more calculating than his manner leads people to believe. Ever since Royal’s 2007 defeat, and even before, Hollande has been laying the groundwork to run for the presidency, Malouines wrote.

But his ambitions were constantly undermined — as they had been from the beginning of his political life — by a general assessment that he was not leadership material. A good assistant, a nice man, a wonderful sense of humor, people acknowledged, but president?

Former prime minister Laurent Fabius, a longtime adversary in the Socialist hierarchy, epitomized Hollande’s image problem in a discussion with political science students in ­Bordeaux in April, just before Strauss-Kahn was eliminated.

“Really, can you imagine Francois Hollande as president of the republic?” he sneered. “In your dreams.”

According to colleagues, it is Fabius who is dreaming now — of becoming Hollande’s foreign minister.