"Modi thinks that I am a stumbling block for him," he told media persons minutes before leaving on his election campaign.
Nitish Kumar was speaking in reference to Modi who hit out against him at rallies in Sasaram and Gaya two days ago, and in earlier rallies in the state and outside.
Modi accused the Nitish Kumar-led government for going soft on terrorists and said that Bihar remained steeped in darkness. He accused the Nitish Kumar government of not doing enough to combat terror.
Nitish Kumar dumped the BJP last year and ended the 17 years old alliance of his Janata Dal-United with the BJP over Modi's rise.
Modi is eyeing Bihar after Uttar Pradesh to get as much of 40 Lok Sabha seats.
The BJP has joined hands with the Lok Janshakti Party of Ram Vilas Paswan, a Dalit leader, and the Rashtriya Lok Samta Party of Upendra Kushwaha, a backward caste leader.