RUTHFULLY YOURS

The lawmakers announced the move in a videoconference Tuesday, saying they planned to caucus in the National Assembly as a new group called “Ecology democracy solidarity.” The group, which totals 17 members, also includes lawmakers that defected from Mr. Macron earlier in his term.

“We are at a historical tipping point, and the decisions that will be taken in the next weeks, months will shape the future of France, of our planet” said Paula Forteza, co-president of the new political group, adding that the government should focus on social justice and the protection of the environment rather than short-term economic policies.

A spokeswoman for Republic on the Move declined to comment. Earlier this month, Republic on the Move’s parliamentary whip Gilles Le Gendre wrote to party lawmakers over the encrypted messaging app Telegram, saying that any defections would represent a breach of trust with Mr. Macron and voters.

“Any attempt to divide us would represent an obstacle to achieve what the French expect from their leaders,” Mr. Le Gendre wrote in the message seen by The Wall Street Journal.

The political rebellion weakens a leader who has long relied on his clout in Parliament to steamroller political opposition and the unions. Mr. Macron’s swagger allowed the former-investment banker to enact sweeping overhauls to France’s labor market and tax code and stare down street protests, the ultimate arbiter in French politics.

Unrest within his own ranks also undercuts Mr. Macron’s efforts to project authority on the global stage. In recent years, he has emerged as the standard-bearer of international cooperation and liberal democracy, leveling critiques at President Trump and other leaders for their nationalist agendas.

Both he and German Chancellor Angela Merkel are facing opposition from Northern European leaders to their proposal to establish a €500 billion recovery fund to support European regions worst hit by the coronavirus pandemic.

Mr. Macron—who once compared his role to that of “Jupiter,” the Roman god of gods—now faces the prospect of governing as a coalition leader for the remaining two years of his term. Modem has 46 seats in the National Assembly, giving Mr. Macron’s coalition a majority of 45 seats.

His government is navigating a political minefield as it tries to enact measures to combat the coronavirus and reopen France’s economy. The government, which has been operating under state of emergency, recently passed legislation to create brigades of health officials to enforce contact tracing and quarantines.

“I do not trust this government anymore,” said Martine Wonner, a lawmaker and former doctor who was booted from Mr. Macron’s party after she voted against the French leader’s plan to ease lockdown rules. She is now part of the new group at the National Assembly.

The rebellion heralds a return to the norm in France’s fractious political system. For decades French presidents from François Mitterrand to Jacques Chirac have presided over unruly coalitions, doling out cabinet posts to appease junior parties and crafting legislation through horse-trading. Major efforts at reform often failed as massive street protests brought successive governments to their knees.

Mr. Macron promised a new brand of politics when he was elected in 2017. His lawmakers were handpicked from across civil society—including doctors, teachers and a renowned mathematician—as well as from lawmakers who defected from the Socialist Party and the conservative Les Républicains.

Mr. Macron eschewed left-right labels, saying his political party and government would be laser-focused on modernizing France’s economy. That was the case early on, as lawmakers who owed their seats to Mr. Macron, rubber stamped one measure after another, including the abolition of a wealth tax created 30 years ago as a symbol of solidarity between the country’s rich and poor. At times, Mr. Macron even circumvented Parliament altogether, decreeing a sweeping overhaul of France’s labor code that made it easier for companies to hire and fire workers.

The pro-business policies earned Mr. Macron the ire of unions and street protesters who branded him the “president of the rich.” It especially alienated members of his party who joined his ranks from center-left parties.

Frustration boiled over in November 2018 as a protest against planned increases in the fuel tax morphed into violent antigovernment riots. Protesters donning yellow safety vests defaced the Arc de Triomphe and stormed government ministries. But Mr. Macron calmed the tensions by going on a listening tour known as the “great national debate.” The format, which packed people into town halls, will be hard to replicate in an era of social distancing.

Write to Noemie Bisserbe at noemie.bisserbe@wsj.com and Stacy Meichtry at stacy.meichtry@wsj.com