"I respect party's decision and accept it. I will never go against it and will work for ensuring victory of the party candidate," Tandon said.
He swept the Lucknow seat in 2009 general elections by 40,000 votes over his Congress rival Rita Bahuguna Joshi.
Rajnath Singh was yesterday declared BJP's candidate from the constituency.
"Its time of change...Roles used to change with time," Tondon, considered close to former prime minister Atal Bihari Vajpayee, added.