The election show on India's state-owned national TV channel Doordarshan was being recorded in a park on Monday in Sultanpur, a town about 160 kilometres (100 miles) from the city of Lucknow.
A man, named by police as Durgesh Kumar Singh, emerged from a crowd of about 150 onlookers, doused himself in petrol and then grabbed the local leader of the Bahujan Samaj Party, Kamruzzama Fauji, engulfing the pair in flames.
Singh sustained burns on 95 percent of his body which are likely to be fatal, while Fauji was in a critical state with 75 percent burns, police said.
"This man suddenly came on the stage, poured petrol on himself and set himself on fire before tightly hugging one of the political guests," local photographer Pankaj Kumar Gupta told AFP.
"People were just too shocked to know what was happening," said Gupta, whose photos show the men engulfed in flames with spectators fleeing the scene.
Two other local politicians taking part in the show, Ram Kumar Singh and Chowdhary Hriday Ram Verma, also sustained minor burn injuries while trying to save the pair.
"The injured were first admitted to the district hospital here (in Sultanpur) and were then referred to Lucknow," said a statement from the police.
The motive for the incident is unknown.
India's numerous television channels are holding nightly debate shows during the ongoing parliamentary elections, often staged in villages and towns with politicians taking questions in front of local voters.
The election, the world's biggest, will end on May 16 with results expected to show the Hindu nationalist opposition Bharatiya Janata Party (BJP) coming to power after 10 years of rule by the leftist Congress party.
Gupta said the show on Monday was called Janmanch 2014 (People's Forum 2014) featuring five local politicians which was coming to an end when the man suddenly appeared.
"Within minutes, the Doordarshan team closed its cameras and packed all their equipment," he said.
