Over the years, Shah, a portly and bearded 50-year-old, has become his closest confidante and key political manager. He is widely expected to take a senior role in any future Modi-run administration.
"Everyone in the party understands that the shortest path to Modi goes via Shah," said one party insider, who spoke to AFP on condition of anonymity.
The married father of a son was in the eye of a storm at the weekend after reportedly telling supporters to see India's parliamentary polls as an opportunity for "revenge" against the left-leaning ruling Congress party.
Speaking in an area torn apart by sectarian hatred last August, he will face a criminal investigation after branding the government as one "that protects and gives compensation to those who killed Hindus".