It is the final frontier for India’s most powerful leader in decades.

Narendra Modi, over his 10 years as prime minister, has made it his mission to turn a complex and diverse country of 1.4 billion people into something approaching a monolith dominated by his sweeping Hindu nationalist vision.

The news media, the national legislature, civil society, sometimes even the courts — all have been bent to his will. But one critical group of holdouts remains: some of India’s richest states, the engine of its rapid growth.

The future shape of the world’s largest democracy — and its economic trajectory — may rest on the power struggle that has ensued.

Mr. Modi, who is well placed to win a third term in a national election that will begin on April 19, is wielding an increasingly heavy hand in what his opponents call an unfair effort to drive out the governments of the states his party does not control.

They accuse Mr. Modi’s administration of delaying federal money for major projects; of jailing or hounding opposition leaders while shielding anyone who joins the prime minister’s party; of obstructing the delivery of basic services; and of throwing state politics into chaos.

The tensions are tearing at India’s delicate federal formula of power sharing and political competition, the glue holding the country together across 28 states and eight territories.