"His (Nitish) opposition to Narendra Modi is just an eye-wash ... He is acting as a tool of L K Advani in the perceived factional fight between Advani and Modi for becoming the PM nominee," Prasad told PTI over phone from Araria district.
Prasad said "Nitish does not have the courage to part company with the BJP."
Nitish is heading a JD(U)-BJP coalition government in Bihar since 2005.
The RJD chief said the talk of secularism by Kumar did not sound convincing as he was "deeply integrated with BJP and RSS.