Janata Dal (S) state chief H Vishwanath on Tuesday resigned from the party post blaming the state Congress for losses in Mysuru and Mandya.
Vishwanath said he takes "moral" responsibility for the party's dismal show in the recent Lok Sabha polls.
"I take moral responsibility for the (party's) defeat," Vishwanath, an MLA, told reporters.
The JDS chief was reportedly unhappy over being sidelined and not being taken into confidence by the party on key issues.
Vishwanath was also locked in a public spat in recent weeks with senior Congress leader Siddaramaiah over his functioning as the ruling coalition coordination committee chief and for not preparing the common agenda for the two partners.
The BJP had swept the Lok Sabha polls in Karnataka, winning 25 of the 28 seats, leaving the ruling coalition partners Congress and JDS with one seat each.
Independent candidate Sumalatha Ambareesh had won the Mandya seat.