He becomes the second nominee to have deserted the party soon after getting ticket.
Tomar, who had unsuccessfully contested the last Lok Sabha polls as a Congress nominee from Gautam Budh Nagar, returned to the BJP fold after a gap of over five years as he was earlier a three-time BJP MP from neighbouring Ghaziabad.
Welcomed to the party fold by BJP chief Rajnath Singh, Tomar said he was "coming back to his home".
"I decided to join the BJP because of Narendra Modi as he has promised good governance and development," he said after joining the saffron party.
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Praising Modi, he said, "Modiji is for development and good governance and I will work for the victory of the BJP in the election."
Tomar also accused Congress of encouraging communalism and divisive politics citing this as a reason for leaving the party.
Prasad had unsuccessfully contested the last Lok Sabha elections on a Congress ticket from the same seat. This time also Congress had relied on him and nominated him from the same seat, but he switched over to the saffron party leaving the party red-faced.