KOLKATA, India — To topple 34 years of communist rule and become chief minister in India’s West Bengal state, Mamata Banerjee put her life on the line for decades.

She built a vast grass-roots base, held hunger strikes and sit-ins that were attacked by the police and jumped back into street protests soon after communist workers bashed her head with metal rods.

So when Prime Minister Narendra Modi’s governing party targeted such a proven survivor, campaigning hard in her state in the parliamentary elections this year, it pulled out all the stops.

Mr. Modi’s Bharatiya Janata Party spent heavily, going after the same villages and districts she once swung against the communists and enlisting political operatives once allied to her to dig up dirt and personal attacks.