(Adam Taylor,Jason Aldag/The Washington Post)

In the country that invented the modern notions of political right and left, Emmanuel Macron won the French presidency from perhaps the most precarious place possible: the center. 

Now he has to figure out how to govern from there. 

Macron’s victory on Sunday as a 39-year-old political newcomer who enjoyed no party backing and faced a wave of populist anger was improbable enough. 

But the task he began confronting Monday is even more difficult. He must figure out how to translate the poetry of a campaign built on borrowing the best ideas from either end of the political spectrum into the prose of governing in a way that doesn’t alienate everyone. 

At stake is not only his presidency and the future of a nation of 67 million, but also mainstream politics across the Western world. By thoroughly defeating far-right leader Marine Le Pen, Macron instantly became a symbol among those in Europe and North America who are seeking a new style of politics that can break the populist fever.

If he fails to govern effectively from the middle, the extremes could soon rise again.

“It’s a huge challenge,” said Dominique Moïsi, a senior adviser at the Paris-based Institut Montaigne, a think tank that is close to Macron. “He’s trying to renew politics, to do it in a different way. I think he can pull it off. But there’s no doubt it will be difficult.” 

Macron will have little time to figure out his strategy. He’s due to be inaugurated Sunday and will barely have the chance to unpack bags at the ElyseePalace before he once again hits the campaign trail. 

This time, Macron will be advocating not for himself but for the 577 representatives of his movement seeking seats in National Assembly elections next month. 

Their fate and Macron’s are intimately linked. Without a parliamentary majority, or something close to it, France’s new president will have little chance of enacting his ambitious agenda. A poor showing in next month’s vote could doom his young government before it really even starts.

Normally, the parliamentary vote that follows a presidential election in France is considered something of a formality — a chance for voters to reiterate their choice and give the president the legislative backing he needs. But not this year. 

After a bruising and fragmented campaign — Macron won less than a quarter of the vote in the first round, and most supporters said they were voting against his opponent rather than for him in the second round — it’s not clear whether voters will want to give him a free hand.

There’s also the problem that Macron’s movement — En Marche, or Onward — is starting from zero, not having existed the last time France chose its lawmakers. 

The party will now have to select its candidates — without overly relying on the entrenched political clique that Macron railed against during his campaign.

“There’s a contradiction in Macronism. He wants to renew the way we do politics. But for now those who support him are politicians who have been in the game for a while,” said Eddy Fougier, a researcher at the Institute for International Relations and Strategies. “He’s trying to make something new out of the old.” 

Fougier said En Marche candidates would likely end up being a combination of familiar political names, as moderate lawmakers from the traditional center-right and center-left parties defect to Macron, and political newcomers from civil society. 

Richard Ferrand, secretary general of En Marche, told journalists on Monday that the party would announce the names of its candidates Thursday. Half will be women, he said, and half will have never held elected office.

Ferrand now represents the Socialist Party in Parliament, but said he would run this year under Macron’s banner.

Macron himself was the economy minister under Socialist President François Hollande. But too many ex-Socialists, Fougier said, would be politically fatal for Macron, who must prove to voters that he’s not “the second coming of Hollande.”

That was the charge that Le Pen leveled at her opponent throughout the campaign. For months, Macron carefully avoided any association with his deeply unpopular former mentor. 

But on Monday it was unavoidable. A raucous celebration among Macron supporters Sunday night — which also served as a pep rally for the European Union — gave way Monday morning to a somber procession on the Champs-Elysees as the nation remembered its war dead on Victory in Europe day. 

Macron and Hollande together laid a wreath, with   the outgoing president making time to pat Macron on the back and offer an apparently heartfelt “Bravo” — a sentiment echoed by leaders from around the world, including Russian President Vladi­mir Putin.

In a message to Macron — who has taken a tough line with Moscow, and whose campaign was the victim of a suspected Russian hacking attack in the campaign’s closing hours — Putin urged the president-elect to “overcome mutual distrust” and wished him “strong health.” 

President Trump congratulated Macron in a phone call Monday, the White House said, and the two agreed to meet at the May 25 NATO gathering in Brussels.

But domestically, the bon­homie of Macron’s win was already giving way to the usual bare-knuckled politics. 

Le Pen, for one, had barely finished conceding defeat in the presidential race Sunday night when she announced her party would be the “primary force of opposition” to Macron’s government. (She did, however, take at least a little time off from politics, shimmying with supporters to the tune of the Village People’s “Y.M.C.A.”)

The remnants of the Socialists will also be gunning for Macron in the parliamentary vote. So, too, will the center-right Republican party, which was widely expected to win the presidency this year before its candidate, former prime minister François Fillon, stumbled badly. Polls show the party is Macron’s closest challenger in the parliamentary vote.

Once lawmakers have been elected, Macron will start pushing the agenda he campaigned on. With anything less than an En Marche majority in the multi­party Parliament, he’ll have to opportunistically seek out allies across the aisle to avoid gridlock.

Macron’s platform includes elements designed to appeal to either side of the political spectrum: smaller class sizes in public schools and a shift to cleaner energy sources to satisfy the left; sharp reductions in government bureaucracy and a more flexible labor system to appeal to the right. 

The latter will be especially important to Macron’s success. The president-elect has said that loosening the country’s notoriously rigid employment system will be key to unlocking greater economic growth and ultimately bringing down chronically high levels of joblessness.

But his push for reform could provoke an early showdown with France’s influential labor unions — as Macron knows well from his earlier tangles with the unions as economy minister. 

Many observers are betting against the new president’s ability to deliver. But Moïsi, the analyst whose think tank is close to Macron, said that especially after the last election, no one should be too quick to count him out.

“He has cards to play: his character, his timing, the fact that the global economy, especially in Europe, is picking up,” Moïsi said.   “The French are politically ready for sacrifices and reforms as long as there’s someone in Elysee Palace with credibility, energy and optimism. There’s a dynamic behind him that may carry him through.” 

Virgile Demoustier contributed to this report.