NEW DELHI—India’s debt problems have spread to consumers, gumming up the most-important piston propelling Asia’s third-largest economy and dimming its chances of snapping out of a slowdown.

India’s gross-domestic-product growth has slowed to a five-year low and is expected to head lower. Its strapped central government can’t spend to help, and banks are struggling and won’t lend much. Now consumers, whose spending makes up more than 65% of India’s economic activity, are turning more stingy. The government is scheduled Friday...