The elections in 59 seats in the 60-member Assembly were held on February 18.
"The CPI-M has done well in the poll". The remaining eight seats went to a regional group, the Indigenous People's Front of Tripura.
Rio was declared as the chief ministerial candidate of the NDPP-BJP alliance by the combine.
On the other hand, the CPI (M)-led Left Front, which had captured 50 of the 60 seats in the 2013 Assembly polls, is expected to secure almost 44 per cent this time, the EC source said.
The Karat faction has so far dominated the party by espousing a "big no to Congress" line and winning a vote in Central Committee last month.
The Congress is not expected win a single seat.
Rio said he was grateful to the people of Nagaland and his party colleagues. It is that the Congress gets satisfied with a "moral victory" - as in Gujarat, before it lost steam - while the BJP is spurred on by setbacks. Why? The party's senior functionary and a minister Assam government, Himanta Biswa Sarma, told reporters that the outgoing chief minister of Tripura, Manik Sarkar could not take shelter in Bangladesh.
A total of 290 candidates, including 23 women, of the ruling CPI-M, CPI, BJP and Congress and many Independents, were in the fray.
The loudest message coming out of Tripura after Saturday's poll verdict is that the BJP can indeed make headway in "virgin territories" - and that too in a short span of time - if it knuckles down. But this year they grabbed 35 seats.
Prime Minister said that BJP is committed for complete change in the political system in the country. The politics of violence has been defeated by the power of democracy in Tripura. The Congress had its voters in the state. "The entire Congress leadership crossed over to the BJP", CPI (M) politburo member Brinda Karat told PTI.
PM Narendra Modi said the result in Tripura's tribal regions had in particular given him great joy since it dispelled notions that these regions were affected by Maoism and Naxalism.
Rio said the National People's Party's Nagaland president Ato Yepthomi expressed willingness to extend support to the NDPP-BJP alliance in the formation of the government.
The state unit also honoured party workers who campaigned for the party in the three states.
During the programmes the state government will inaugurate 116 educational institutions, water supply schemes and playgrounds.
He said the NPF has asked the BJP to clarify whether it has broken the alliance or it is with the NPF but there was no any reply. "Our two main priorities are the (resolution of) political issues and the other one is development agenda of our honourable Prime Minister", Rio said. Even so, it seems impossible to resolve such a crisis by election time next year.
Chief minister N. Biren Singh announced the week-long celebration of the BJP's achievement in Tripura, Nagaland and Meghalaya, during a party programme organised here on Sunday as a part of the nationwide celebration of the party's performance in the three states.
"It is also very good to have a very strong opposition in a healthy democracy".
Victory in the northeast will not dilute popular disenchantment with BJP rule in Rajasthan. "I bow to every BJP Karyakarta (worker) for working assiduously on the ground for years", Modi said.
The Tripura story is as much about a Congress collapse as it is about the Left's growing irrelevance.