Senior Congress leader Abhishek Manu Singhvi said the aim of the rainbow alliance was to defeat the BJP and form a secular government, while speaking at Mamata Banerjee's big political meet in Kolkata.
Leaders of most big opposition parties except the Biju Janata Dal (BJD) and the CPI(M)-led Left Front, are participating in the rally.
"Never before have we seen such vendetta politics. When Amit Shah held 100 rallies in Uttar Pradesh, no one asked a question. But when RJD organised a rally, they got an Income Tax notice," he said.
Congress is an ally of the Lalu Prasad's RJD in Bihar.
Accusing the government of indulging in "cheap politics", the Congress leader said the biggest beneficiary of vote division in the nation is BJP.
"Prime Minister Narendra Modi makes fun of the opposition alliance, but his party made the most unethical alliance in Kashmir. What will he say about it?" Mr Singhvi said.