"To ensure unity in diversity, equal rights for women and a unified India, I will campaign for the party," Karthik told reporters here.
Tamil Nadu Congress Committee President B S Gnanadesikan said the party "could not field" Karthik as candidates were already announced by it.
Karthik, as part of the BJP alliance, had unsuccessfully contested in Virudhunagar constituency and garnered 2.26 per cent votes in 2009.
Karthik's announcement comes at a time when almost all political parties have a "star cast" to campaign for them except Congress.