"RJD's existence is under threat. Therefore, Tejashwi's statements have no meanings for me," Kushwaha said, adding that it's in the national interest that Narendra Modi should become the prime minister again in 2019.
Taking to Twitter, Tejashwi, who is also the leader of opposition in Bihar Assembly, said the BJP-led NDA has ignored Kushwaha since the alliance came to power in 2014.
"We invite Union Minister Upendra Kushwaha to be a part of the Mahagathbandhan. He is being ignored by the NDA for the last four years. He is being subjected to step-brotherly treatment by the BJP. Also, BJP colluded with Nitish Kumar and conspired to break his party," Tejashwi said in a series of tweets.
Tejashwi said Kushwaha represents a large social group but none from that group was made cabinet minister (in the NDA regime).
"Capabilities of Kushwana, who comes from backward class, were not recognised by the BJP," he said.
Tejashwi urged the RLSP chief to take the "appropriate decision" soon to join the "fight to save the Constitution."
"Upendra Kushwahaji come from the stream of social justice so he will have to choose either the stream of Godse and Golwalkar or of Gandhi and Ambedkar," he said.
"The BJP is abolishing the constitution, hence the reservation will automatically get abolished," he alleged.
According to sources, Kushwaha was not happy with the NDA over the seat-sharing formula in Bihar for the 2019 Lok Sabha elections.
He was also was absent at a get-together of the NDA last week and his party demanded that the next Lok Sabha and assembly polls be fought under his leadership.
The RLSP has been viewing with consternation the assertion by Chief Minister Nitish Kumar's JDU that it was the main party in the NDA in Bihar, a clear hint that it wanted the largest number of seats among the constituents of the BJP-led coalition in the 2019 Lok Sabha polls.