The attack happened on Sunday night in Meghalaya’s East Garo Hills district, about 250 kilometres (150 miles) from the state capital Shillong.
The candidate for the Nationalist Congress Party from the Williamnagar assembly constituency, Jonathone Sangma, was killed along with his driver and two police.
“The vehicle in which Sangma was traveling bore the maximum impact of the explosion. Everyone in the car died on the spot,” said Ringrang Momin, a senior state police officer.
Police officials said there were four active militant groups in the area demanding secession from India for the state bordering Bangladesh.
“This is the handiwork of the militant groups… We have yet to identify the outfit involved in the landmine explosion,” Momin said.
Elections for the 60-member house in Meghalaya are scheduled for February 27.