Under Article 355 of the Constitution of India, it is the duty of the Union to :

Article 355 of the Constitution of India explicitly imposes a dual duty on the Union: to protect every State against external aggression and internal disturbance, and to ensure that the government of every State is carried on in accordance with the provisions of the Constitution [2]. This article serves as the justification for the Union's intervention in state affairs under Article 356, commonly known as President's Rule, when the constitutional machinery fails [1]. In contrast, the extension of executive powers and the exercise of state legislative powers by Parliament are consequences of a proclamation under Article 356, rather than the primary duty defined in Article 355 [2]. Furthermore, the power to include or exclude castes or tribes from the Scheduled lists is governed by Articles 341 and 342, involving the President and Parliament, and is unrelated to the emergency duties of Article 355 [3].

Sources

1. [1] Indian Polity, M. Laxmikanth(7th ed.) > Chapter 17: Emergency Provisions > Grounds of Imposition > p. 178
2. [2] Laxmikanth, M. Indian Polity. 7th ed., McGraw Hill. > Chapter 17: Emergency Provisions > Grounds of Imposition > p. 178
3. [3] Introduction to the Constitution of India, D. D. Basu (26th ed.). > Chapter 11: The Union Executive > Parliamentary safeguard. > p. 225