The Election Commission of India (ECI) was boxed in from multiple sides. On one side, it was weighed by the constitutional authority and powers bestowed on it under Articles 324 and 172 (1) of the Indian Constitution. This makes it obligatory for the Commission to hold elections within the tenure of every Vidhan Sabha, which can continue for five years since its constitution, but “no longer.”
While the terms of Assemblies in Punjab, Uttarakhand, Manipur, and Goa end in March, the life of the Uttar Pradesh Vidhan Sabha is till May. However, as per past practice, polls in a bunch of states are held simultaneously whenever Vidhan Sabhas complete their terms around the same time.

