BERLIN — Last December, as he was plotting what most considered to be a hopeless bid to become Germany’s next chancellor, Olaf Scholz interrupted his campaign preparations for a video call with an American philosopher.

Mr. Scholz, a Social Democrat, wanted to talk to the philosopher, Prof. Michael J. Sandel of Harvard, about why center-left parties like his had been losing working class voters to populists, and the two men spent an hour discussing a seemingly simple theme that would become the centerpiece of the Scholz campaign: “Respect.”

On Wednesday, Mr. Scholz will be sworn in as Germany’s ninth postwar chancellor — and the first Social Democrat in 16 years — succeeding Angela Merkel and heading a three-party coalition government. Defying polls and pundits, he led his 158-year-old party from the precipice of irrelevance to an unlikely victory — and now wants to show that the center-left can again become a political force in Europe.

Mr. Scholz won for many reasons, not least because he persuaded voters that he was the closest thing to Ms. Merkel, but his message of respect resonated, too. For the first time since 2005, the Social Democrats became the strongest party among the working class. Just over 800,000 voters who had abandoned the party for the far left and far right returned in the last election.

“Scholz touched a nerve,” said Jutta Allmendinger, president of the research institute WZB Berlin Social Science Center and an expert on inequality who has known Mr. Scholz for almost two decades. “Many see him as a Merkel clone,” she noted. “But he is a Social Democrat to the core.”

Mr. Scholz served as finance minister in Ms. Merkel’s conservative-led coalition government and has promised continuity and stability. Yet he also intends to make Germany a political laboratory of sorts, to try to repair the bridge between the Social Democrats and the working class, an effort with parallels to President Biden’s political agenda in the United States.

For the center-left in Europe, Mr. Scholz’s victory comes at a critical moment. Over the past decade, many of the parties that once dominated European politics have become almost obsolete, seemingly bereft of ideas and largely abandoned by their working class base.

The political energy has been on the right, especially the populist far right, with many American conservatives flocking to countries like Hungary to study the “illiberal democracy” of Viktor Orban, that nation’s far-right prime minister.

“Everyone is looking at us,” said Wolfgang Schmidt, Mr. Scholz’s longstanding adviser, who he has picked to head the chancellery. “If we do things right, we have a real chance. We mustn’t make mistakes, we mustn’t disappoint expectations.”

In her final years in office, Ms. Merkel, a conservative, was at times regarded as the lone defender of liberal democracy in an age of global strongmen, whether President Vladimir V. Putin of Russia or President Donald J. Trump. Yet Germany was not immune to populist fury, and the Alternative for Germany, or AfD, won seats in Parliament and became a political force in the country’s east.

“The biggest concern in politics for me is that our liberal democracies are coming increasingly under pressure,” Mr. Scholz says about himself on the Social Democrats’ website. “We have to solve the problems so that the cheap slogans of the populists don’t catch.”

Mr. Scholz has traveled extensively in the United States, including in the years before the 2016 election. One of his advisers recalled that in a private conversation he even predicted a Trump victory. Then he spent months analyzing why the Democrats lost and reading a raft of books by authors from working class backgrounds in the United States, France and Germany.

“He studied very carefully what happened in the United States,” said Cem Özdemir, a prominent member of the Greens and a minister in Mr. Scholz’s incoming government. “He studied the losses of the Democrats in the U.S. Why didn’t Hillary win?”

When Mr. Scholz’s own party collapsed in the 2017 election, losing for the fourth time in a row, he wrote an unsparing paper concluding that one reason the Social Democrats had lost their core voters was because they had failed to offer them “recognition.”

Last year, in the middle of the first Covid-19 lockdown, Mr. Scholz read Professor Sandel’s latest book, “The Tyranny of Merit” in which the Harvard philosopher argued that the meritocratic narrative of education as an engine of social mobility had fueled resentment and contributed to the rise of populists like Mr. Trump.

“The backlash of 2016 vividly expressed that simply telling people, ‘You can make it if you try’ was not an adequate response to the wage stagnation and job loss brought about by globalization,” Professor Sandel said in an interview. “What Social Democratic elites missed was the insult implicit in this response to inequality, because what it said was, ‘If you’re struggling in the new economy, your failure is your fault.’”

During the last Social Democratic government in Germany, the chancellor, Gerhard Schröder, cut benefits and undertook a painful overhaul of the labor market between 2003 to 2005 in a bid to bring down a jobless toll that had surpassed five million. Mr. Scholz, then the party’s general secretary, became the public face of the changes.

Unemployment did gradually fall, but the program also helped create a sprawling low-wage sector and prompted many working-class voters to defect from the Social Democrats.

Professor Sandel argues that it was around this time that center-left parties, including the Democrats of President Bill Clinton, embraced the market triumphalism of the right, became more closely identified with the values and interests of the well-educated and began losing touch with working-class voters.

Mr. Scholz, once a fiery young socialist who joined his party as a teenager, defended workers as a labor lawyer in the 1970s before gradually mellowing into a post-ideological centrist. Today he is considered to be to the right of much of the party’s base, not unlike Mr. Biden, with whom he is sometimes compared, even though, like Mr. Biden, he has demonstrated some liberal reflexes.

“He was an idealist in his youth, then became a technocrat and even a hyper-technocrat, but I think he might be getting more radical again, at a more advanced age,” said Kevin Kühnert, a prominent figure in the Social Democrat’s left wing who is the party’s new general secretary.

During the pandemic, Mr. Scholz, then finance minister, impressed critics on the left when he unleashed hundreds of billions of euros in state aid to help struggling workers and businesses. The pandemic, in turn, highlighted how those suddenly deemed essential — nurses and social care workers, but also trash collectors, supermarket cashiers and delivery workers — often do not earn very much.

“The pandemic has shown whose shoulders our society is built on, who works hard and still benefits too little from an economic upswing,” Mr. Scholz told reporters on the campaign trail.

Mr. Scholz will now lead a three-party government with the progressive Greens and the libertarian Free Democrats. Their governing treaty calls for raising the minimum wage to 12 euros, or about $13.50, an hour, from €9.60 today — an instant pay rise for some 10 million people. Mr. Scholz has also promised to build 400,000 homes a year, 100,000 more than was previously planned, and to guarantee stable pension levels.

More abstract, but equally important, is his promise of another “industrial revolution” that will aim to make Germany a manufacturing power for the carbon-neutral age and provide the economic bedrock for the welfare state of the future.

“We need to tell people two things,” Mr. Scholz said during the campaign. “First, that we need respect, we need good pay and proper recognition for work. And second, we have to ensure that there are good jobs in the future.”

Across the European Union, Social Democrats govern in nine of the 27 member states, and lessons from Germany are already proving influential. In France, the Socialist mayor of Paris, Anne Hidalgo, who recently announced her own long-shot presidential bid, has evoked the “respect” theme.

But slogans only go so far. The Social Democrats came in first in the splintered September vote in Germany, but mustered only 26 percent of the total, a far cry from the 40 percent they recorded at the start of Mr. Schröder’s first term. Mr. Kühnert, the party’s general secretary, said that Mr. Scholz’s challenge is to show that the Social Democratic model is the right approach for the country and beyond.

“We hope that our election victory in Germany will send a signal for the revival of social democracy internationally,” Mr. Kühnert said. “We’re looking above all to the rest of Europe. Because we need to strengthen the E.U. in the next years if we want to have anything to say in the world in coming years.”

Christopher F. Schuetze contributed reporting.