French President Emmanuel Macron's strategic sacrifice, giving up an effort to make age 64 the baseline for full retirement benefits, could be the move that finally allows him to push through reforms that have bedeviled governments for a quarter-century.

After more than a month of strikes that have crippled public transport, a key early sign appears to be in his favor. Disruption to train services in France decreased Monday as some union leaders tentatively embraced the new plan.

That may boost broader public support for the proposed reforms as some unions and employer representatives look to hash out alternate ways to ensure a balanced budget for the retirement system over the next three months.

Pensions are seen as the mother-of-all reforms in France and several previous governments have backed down in the face of public opposition to their attempts to change it. Mr. Macron's government successfully barreled through changes to tax and labor laws with little effective opposition.

But the gridlock that forced the government to back down on one key measure shows how deeply the French are wedded to their pension system.

Mr. Macron aims to merge 42 separate regimes into a single universal points-based program that would, over time, save money and help balance the retirement system's budget. A hot-button proposal was the now-abandoned effort to raise the age for full retirement benefits, the so-called pivot age, to 64 from 62.

"Everyone has moved, but balancing the budget, the principle of it and the means to get to it, that's not negotiable," Prime Minister Edouard Philippe said Sunday on France 2 television.

In a letter to unions Saturday, Mr. Philippe set out plans for a conference with stakeholders to hammer out how to finance the country's creaking retirement system, in exchange for "provisionally" dropping the pivot age. He cautioned that if the sides fail to reach an agreement, the government will issue new rules by decree to bring financing of the system into balance by 2027.