Pathak joined the saffron organisation in the presence of its president Amit Shah and Union minister Mahesh Sharma and was described as a "pillar" in the Hardoi-Unnao region of the state. Assembly polls in UP are scheduled next year.
Sharma said Pathak joining the BJP will help the party consolidate Brahmin votes.
Even though he was at the BJP headquarters fulfilling his joining formalities, the BSP expelled him for the party, apparently after coming to know of his plans.
Unlike some BSP leaders, including Swami Prasad Maurya, who tore into BSP supremo Mayawati as they joined the BJP, Pathak did not attack her but said nepotism, corruption and goondaism ruled the roost in the state.
"I thought any delay will be harmful. Only Prime Minister Narendra Modi's developmental agenda and Amit Shah's efforts can rescue UP," he told reporters.
Asked about his presence in the BSP rally on Sunday and he joining the BJP is less than 24 hours, he said he was doing his job in the BSP as long as he was there.
Pathak said he had played a role in swinging Brahmin voters to the BSP during the 2007 assembly polls, but Brahmins were disappointed by the party.
The BJP is counting on the former Unnao Lok Sabha MP and now a sitting Rajya Sabha member, to help it win over Brahmin voters, who are also being wooed by Congress.
IMAGE: BJP Chief Amit Shah with Brajesh Pathak. Photograph: @dr_maheshsharma/Twitter
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