The ad, issued by the Bharatiya Janata Party, asks why Mr Kumar has not acted on "repeated insults" to the cow by his allies. "Stop vote bank politics and explain whether you endorse these statements," the ad says, before listing three comments made by Rashtriya Janata Dal chief Lalu Prasad and others.
In each, the word beef has been highlighted in red.
A furious anti-BJP 'Grand Alliance' has complained to the Election Commission. "BJP's ads show how insecure they are... such comments, ads haven't appeared in the last 40 years... If the election process isn't fair, people will lose faith in democracy," said KC Tyagi of Nitish Kumar's Janata Dal (United).
The Election Commission has now asked all parties to not publish any political ads in papers tomorrow without its clearance.
Delhi Chief Minister Arvind Kejriwal shared the ad on Twitter and said, "Have these ads in Bihar been given by BJP's fringe elements? Or by BJP top brass?"
Senior BJP Bihar leader Sushil Modi wondered what was wrong with "seeking answers from Nitish Kumar," and said, "They (grand alliance) stoked the beef issue in Bihar."
"Don't Hindus eat beef too? It is the poor who are forced to eat beef. People eat (beef) outside the country too. What difference does it make?" Lalu Prasad had said soon after a Muslim man was killed by a mob over rumours of beef eating and cow slaughter in neighbouring Uttar Pradesh.
The BJP immediately launched an aggressive campaign around the remark, with Prime Minister Narendra Modi repeatedly attacking Lalu Prasad in election speeches.
Bihar has already voted in four phases in what is seen as a very close contest. The fifth phase tomorrow will see voting in 57 constituencies. Votes will be counted on Sunday, November 8 for all 243 seats.
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