Francois Mori / AP Supporters of François Hollande react after hearing presidential-election results outside the Socialist Party campaign headquarters in Paris on Sunday, May 6, 2012

France was poised for major change Sunday night when Socialist candidate François Hollande looked set to beat conservative incumbent Nicolas Sarkozy in runoff balloting for France’s presidency. Initial results revealed at 8 p.m. local time forecast Hollande beating Sarkozy by a seemingly insurmountable score of 51.9% to 48.1%. When that result is confirmed by the final tally, Hollande will become France’s first Socialist President in 17 years — lifted to power by a majority of voters who embraced his calls for a greater emphasis on economic growth and targeted social spending to counteract the austerity measures implemented to help shrink France’s excessive debt. Yet in addition to reflecting public desire for policy change, Hollande’s election will also be interpreted as a personal rebuke to the unpopular and embattled Sarkozy, who becomes France’s first single-term President in 31 years.

The announcement of Hollande’s projected win was greeted with an eruption of cheers by elated supporters gathered around the country — including a festive throng assembled outside the Socialist Party’s Left Bank headquarters in Paris. Chants of “François President!” and “We’ve won!” rang out to celebrate the news awaiting Hollande’s televised victory speech from the south-central town of Tulle, which he has represented as a regional official and national legislator since the late 1980s. Socialist officials said they expected Hollande’s comments to take up his campaign theme of unity to deal with the financial crisis that France and Europe face in contrast to what he had said was Sarkozy’s penchant for division. Less than 20 minutes after the partial results were announced, Sarkozy addressed his supporters in a Paris auditorium, telling them that he “assumed full responsibility for this defeat.” He also wished Hollande luck in the “trials” facing France’s top leader at a time of continental crisis.

Hollande’s runoff triumph ended what had become an increasingly nasty duel with Sarkozy in the wake of first-round voting on April 22. Rather unexpectedly, Hollande topped the field of 10 candidates in that initial stage with 28.6% of the vote, slightly edging Sarkozy’s 27.2%. That result — the first in Fifth Republic history in which a participating incumbent failed to win the opening round — compounded the challenge that Sarkozy already faced in reversing simulated runoff-voting results that for months have shown Hollande winning handily.

In his efforts to do that, Sarkozy not only attacked his Socialist rival as being a “liar” whose leftist program would “bring France to ruin” amid the euro crisis. He also stepped up his much decried bid to lure over 6.4 million people who voted for National Front leader Marine Le Pen on April 22 by taking up nationalistic, protectionist and anti-immigrant positions traditionally defended by the extreme-right — and scorned by France’s mainstream. In the end, that strategy clearly proved more damaging than helpful to Sarkozy’s re-election fortunes.

With the caustic campaign battle finally over, many Hollande backers were less focused on recrimination and more intent on celebrating the left’s first presidential win since Socialist François Mitterrand left the Elysée in 1995. In anticipation of Hollande’s triumph, preparations were made earlier Sunday at the Place de la Bastille to welcome a crowd that had gathered as of late afternoon to replicate the giant party that formed there in 1981 to hail Mitterrand’s upset presidential victory over incumbent Valéry Giscard d’Estaing (Sarkozy’s one-term predecessor). Hollande himself was scheduled to fly from Tulle to Paris in the evening and join Socialist officials and supporters in the Bastille area to mark his success.

The fete promises to be short-lived, however. With Sarkozy’s term set to end on May 15 at the latest, Hollande must move quickly to appoint an interim leftist government awaiting legislative elections in June. He will also have to lay out economic policy priorities that voters, his euro-zone colleagues and financial markets are all anxiously awaiting to hear — and will doubtless respond to in differing ways.

Hollande must also prepare for a summit of NATO leaders in Chicago in June. Before that, Hollande has said he’ll organize a visit to Berlin to meet with austerity-focused German Chancellor Angela Merkel. Merkel has voiced loud opposition to Hollande’s promise to renegotiate the E.U. deficit- and debt-fighting treaty to add growth mechanisms to it. She even publicly endorsed the re-election bid of her treaty co-instigator, Sarkozy — at one point pledging to campaign in France on behalf of the President.

Hollande has said he won’t allow Merkel’s actions in favor of Sarkozy to undermine Franco-German relations. Indeed, given the importance of that partnership in European affairs — and his own determination to renegotiate the treaty critical to the euro’s future despite the Chancellor’s hostility — Hollande is expected to make contacting Merkel one of his first moves as President-elect.

“I think François Hollande will speak with the German Chancellor this very evening,” said Socialist official and potential Prime Minister Jean-Marc Ayrault, according to French press reports on Sunday. “Because [that partnership is] vital to Europe’s recovery, [and] the reorientation of Europe toward growth, toward competitiveness and toward protection.”

Wooing the conservative Merkel away from the total-austerity remedies she favors and toward a campaign to invest in economic growth won’t be easy for a President Hollande who has no experience in national government or international relations. But if he managed to overcome what at one time looked like impossible odds to defeat Sarkozy, who knows what else might be possible?