Mr Kumar was alluding to Mr Modi having refused to wear a skull cap offered to him by a Muslim cleric two years ago.
Nitish Kumar today used a lecture on his "Idea of India" to hand some unsolicited advice to Narendra Modi, the BJP's prime ministerial candidate.
He did not have to name Mr Modi when he said, "To run the country, you have to take everyone along... at times you will have to wear a
topi (cap), at times a
tilak (vermillion mark on the forehead)."
The Bihar Chief Minister was alluding to Mr Modi having refused to wear a skull cap offered to him by a Muslim cleric two years ago. Mr Kumar questions Mr Modi's secular credentials and accuses the Gujarat Chief Minister of not having done enough to prevent the communal riots of 2002 in his state, in which hundreds of Muslims died under his watch.
Mr Kumar has attacked Mr Modi often, but this was the first time since the latter was picked to front his party in the 2014 general elections a week ago.
In his lecture at a function organised by the National Commission for Minorities, there were other digs at Mr Modi. "A wave is being forced through a blower in India," he said, seemingly in response to leader after leader of the BJP claiming that there is a Modi wave in the country.
Three months ago, Nitish Kumar broke his relationship of 17 years with the BJP because it chose Mr Modi to head its campaign committee.
Since then there has been much speculation about a possible alignment of his party, the Janata Dal (United), with the ruling Congress as the time for elections draws closer.
Today, he seemed soft on the Congress-led Centre when he said, "The GDP can be high for many reasons, the question is how safe are people feeling, has their life improved or not? You have to be worried about health, education."
The Congress has been attacked repeatedly by the Opposition on the fragility of the Indian economy.
Mr Kumar's party supported the Congress in pushing its flagship welfare schemes in parliament in the monsoon session. Schemes like those on food security and land acquisition are expected to be big vote-getters for the ruling party.