PARIS — By almost any measure, the party of President Emmanuel Macron achieved overwhelming success in the first round of parliamentary elections on Sunday.
The candidates of his newly formed party, La République en Marche, finished first in 449 of 577 districts, leaving them poised to dominate the National Assembly, the lower and more powerful house of French Parliament, after the second round of elections next Sunday.
That success built on Mr. Macron’s strong early performance as president, analysts said, but was also greatly helped by the vacuum left when successive parties on the left and right collapsed in the face of his strong showing in the presidential race and by historically low turnout — just 49 percent of the French went to the polls.
The combination of factors has left some analysts and historians wondering if perhaps Mr. Macron is even succeeding too well. The vulnerability inherent in his success is that while he will be able to push through his agenda, he will lack a broad base of support because only one in two eligible French citizens voted and his party’s likely crushing majority in the Parliament will overwhelm opposition voices.
In addition, because he is expected to have such a large margin in the National Assembly, his program could win approval with little resistance, allowing him to skip the step of assembling a broad-based coalition. That could come back to haunt him, leaving some, perhaps even many, feeling disenfranchised.
“There will be rather weak political opposition within the Parliament, but we are going to face it on the street, on the social networks, outside of institutions,” said Jean Garrigues, a historian at the University of Orléans.
“And it is always dangerous when political opposition hardens outside of institutions,” he said.
The left-leaning newspaper Liberation on Monday likened Sunday’s results to a “takeover” of the nation’s politics.
None of that undercuts Mr. Macron’s formidable political skills or that of his party, La République en Marche (The Republic on the Move). He managed to take France’s rather querulous desire for change and infuse it with a sense of optimism, with the idea that people could be better off.
Past presidents who had proposed changes in labor laws and the French social safety net had not been able to convince the public that the benefits would outweigh the pain.
“People are wondering what kind of fairy dust he used to make this happen,” said Édouard Lecerf, global director for polling and research for Kantar Public.
Since 2002, when the timing of the French legislative elections changed so that they directly followed the presidential elections, the ballot has served as confirmation of the president’s win, reliably sending a majority of representatives of the president’s party to Parliament. Although pundits initially expressed doubts that Mr. Macron could secure a majority because of how new his party and its candidates were, he helped ensure that outcome by quickly impressing the French during his first days as president.
“He had series of impressive international events with the NATO summit, the G-7 and the meeting with Putin at Versailles,” said Bruno Cautrès, a political scientist at the Center for Political Research at Sciences Po in Paris.
He was referring to Mr. Macron’s star turns both in Brussels where he met President Trump and won the ‘I can shake hands harder than you can’ competition, and at Versailles where he went toe to toe with President Vladimir V. Putin of Russia, bringing up sensitive topics like the treatment of gay people in Chechnya during a joint news conference.
Mr. Cautrès said the new president’s shrewd choices of people, from both the left and right, to fill the ranks of his government had also helped.
Mr. Macron has benefited from the weakness of the other parties; even those parties that were relatively strong as recently as the presidential election have seen a sudden drop in popularity. For instance, the far-right party of Marine Le Pen, the National Front, received 21 percent of the vote in the election’s first round, and the far-left party of Jean-Luc Mélenchon, France Unbowed, took close to 20 percent.
Both lost ground during the legislative contest. Ms. Le Pen’s party got about 13.5 percent of the vote nationwide in the weekend elections, a precipitous drop, and France Unbowed did not quite reach 11 percent.
“The National Front was unable to capitalize on the 11 million votes it won in the presidential election,” Mr. Cautrès said. “The far right appears divided and Marine Le Pen’s leadership could be challenged if she does not win her legislative race on Sunday.”
Ms. Le Pen won 46 percent in a district in the northeast of France where she is running, making it likely that she will win. Less clear is whether more than a bare handful of other National Front candidates will garner the votes necessary to get into Parliament.
On the left, Mr. Mélenchon, who by dint of his personality and debating skills was a strong presence in the presidential election, was unable to project himself into the scores of races nationwide where his candidates were competing.
The Socialist Party of Mr. Macron’s predecessor, François Hollande, fared badly in the legislative elections. It had been accused of betrayal by unions and its own left wing after the government pushed pro-business changes to the labor laws that it had once shunned.
About 100 Socialist Party representatives and their allies in the National Assembly lost their seats on Sunday, including Jean-Christophe Cambadélis, the head of the party; the Socialists’ presidential candidate, Benoît Hamon; and a number of Mr. Hollande’s former ministers.
“It paid dearly for its contradictions,” Mr. Cautrès said.
But the most potentially dangerous element for Mr. Macron is one that helped his party do so well: a historically high abstention rate — 51 percent of the French decided not to vote last Sunday.
The low turnout helped Mr. Macron’s candidates by reducing their competition, but the darker side is that many workers and poorer people in cities as well as in the countryside will not be represented, several historians and political sociologists said.
“There’s a spectacular underrepresentation of the National Front and of the France Unbowed party,” Mr. Garrigues said.
“It is the case that this National Assembly is going to represent the France that is favored to the detriment of the France that is suffering,” he said.
Myriam Revault d’Allonnes, a philosopher and specialist in political representation, sees in the abstention a form of protest rather than apathy. “You don’t abstain because you prefer to go fishing,” she said.
It is not a only a sign of a lack of interest in politics, she said in an interview published on the Franceinfo news website, “it is also a sign of protest.”
“The state of grace will not last forever,” said Mr. Lecerf, the pollster. “Once he starts changing the pensions and work laws, it is going to get much more complicated.”