The Congress leader sent in his resignation to party President Sonia Gandhi, sources close to him said.
Siddaramaiah had been touted as one of the key emerging leaders of the Congress after successfully leading the much-publicised state-wide 302 km padyatra against BJP rule.
Congress nominee K M Kemparaju lost the Kadur seat to BJP's Y C Vishwanath while in Gulbarga South Ajay Singh, son of former Chief Minister Dharam Singh lost the seat to JD-S' Aruna Patil.
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