The BJP has long predicted it would retain the state because of policies such as free staples for the poor during the pandemic, a crackdown on crime, and its popularity among the Hindu majority reinforced by the construction of a temple on the site of a razed mosque.
In elections in four smaller states over the past month, the Aam Aadmi Party that governs the national capital territory of Delhi is headed for a landslide victory in Punjab, while the races are tight but in favour of the BJP in Manipur, Goa and Uttarakhand.
Aam Aadmi Party leaders said they were ready to take on Modi nationally.
It has long been assumed in Indian politics that without winning Uttar Pradesh and the neighbouring state of Bihar, no party or coalition has much hope of securing a majority in parliament. The BJP has been in power in both.
For decades, Uttar Pradesh was a stronghold of the main opposition Congress party but it has been unable to stem a slide in its popularity over recent years.
A win in Uttar Pradesh would be a seal of approval for Hindu monk Yogi Adityanath, who was surprisingly chosen as chief minister for the state five year ago and is seen as a future prime ministerial candidate for the BJP.