Stakes are high for the main opposition Congress, as it is fighting to retain these six seats.
The state leadership of BJP and Congress has mostly stayed away from campaigning. Chief Minister Narendra Modi did not campaign in any of the constituencies.
Voting will be held on June 2 and counting will take place on June 5.
The two Lok Sabha seats where the by-polls are being held are Porbandar and Banaskantha, while the Assembly by-polls would be held at Dhoraji, Jetpur, Limbdi and Morva Hadaf.
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Porbandar MP Vithhal Radadia first resigned when he got elected as an MLA from Dhoraji on Congress ticket in the December Assembly elections.
Later, both Vithhal Radadia and his son Jayesh who had won from Jetpur Assembly seat resigned as Congress MLAs to join BJP. Now father and son are contesting respectively from Porbandar Lok Sabha and Jetpur Assembly seats on BJP tickets.