How did the Congress party expand its social base over time?

Through civil disobedience movements

The Congress party evolved from a pressure group for the educated, professional, and commercial classes in 1885 to a mass movement in the twentieth century.
Its initial composition was dominated by English-speaking, upper-caste, upper-middle-class, and urban elites.
However, with each civil disobedience movement, the Congress party expanded its social base and brought together diverse groups with often contradictory interests.
It included peasants, industrialists, urban and rural dwellers, workers, owners, and people from various social classes and castes.