Whether Mr. Berger follows words with actions remains to be seen. The C.F.D.T., as the union is known, recently became France’s largest when its membership surpassed the militant General Confederation of Labor, which has dominated the landscape for decades.

Amid Mr. Macron’s rise, Mr. Berger has sometimes painted his union as a moderating influence in France’s labor movement at a crucial moment for the economy. Mr. Berger has pushed for a more flexible approach in France as the forces of globalization change the competitive landscape.

Yet he is not immune to protesting when the stakes are high. On Sunday, he urged Mr. Macron to maintain discussions with unions and employer groups — or risk new demonstrations.

In France, even small changes tend to rile labor organizations, which have historically sought to secure workplace protections through protests and strikes. The General Confederation of Labor, known as the C.G.T., has been at the forefront of mobilizing frequent, sometimes violent actions, whether burning tires or even holding bosses captive.

And while unions are at their weakest membership levels ever — representing just 8 percent of the work force — they can still thwart big changes to totems like the length of the workweek, or measures that would undermine their own power.

Mr. Macron’s plans contain several elements that unions, including the C.F.D.T., see as red lines. Foremost is a proposal to allow employers to negotiate directly with employees on a range of workplace issues, overriding sector-wide accords struck by unions. Labor organizations also oppose a measure to cap compensation awards in unfair dismissal cases.