MEXICO CITY — All is not well in the kingdom of Carlos Slim.

For more than 25 years, he has dictated the terms of Mexico’s telecommunications industry and built an empire, making him one of the world’s richest men.

Mr. Slim and his family are billionaires 50 times over. He has stood at the very top of the Forbes World’s Billionaires list — more than once. His flush years in Mexico enabled him to span the Americas with companies that touch nearly every facet of modern life: telecom, banking, construction, retail and media, among others.

But at home in Mexico, the game is changing. And there is not much he can do about it, analysts say.

Determined to bring his dominance to an end, leaders from Mexico’s three biggest political parties have put aside their own animosities in recent years, meeting in secret sessions to chip away at Mr. Slim’s domain.