The communist movement in the country could not be written off because of an electoral setback.
The election for the Charilam ST reserve constituency was countermanded due to the death of CPI-M candidate before the elections.
The BJP has managed to beat the grand old party, once known for its deft political outmanoeuvring, at its own game. The state is likely to have a non-Congress government, as the UDP extended its support the National People's Party (NPP).
BJP leaders have hailed the verdict and said it is a precursor to what will happen in Karnataka and other parts of the country.
The latest poll performance emboldens the BJP to try to oust opposition parties in other states, including Karnataka in southern India and West Bengal, situated next to the Northeast.
NDPP secretary-general Abu Metha said the coalition is proposed to be named the People's Democratic Alliance.
Taking about the victory in the state of Tripura and Nagaland, Prime Minister Modi said that now the time has come when the election analysts in India need to understand the journey from "No one to One".
The differences are on key issues such as the bifurcation of Tripura and Meghalaya, Congress communications in-charge Randeep Surjewala said in a series of tweets as the BJP was set to form government with its partners in the northeast states of Tripura, Meghalaya and Nagaland.
With an eye on winning Left-ruled Tripura, Sunil Deodhar, who did a good job in Varanasi during Modi's Lok Sabha campaign, was sent to the state in 2014 itself.
Deodhar, who had been an RSS Pracharak in Meghalaya for 1991 to 2001, was familiar with the socio-political dynamics of the North East states.
The NDPP got 17 seats while the BJP got 12. "I bow to every BJP Karyakarta (worker) for working assiduously on the ground for years", Modi said. "They supplied oxygen to the BJP", she revealed.
"Both states have one fact in common: the left (communist) rule for decades that had made the cadres apathetic and nearly bourgeois in their approach to government and politics", said Seema Mustafa, a political analyst who runs the Centre for Policy Analysis think-tank in New Delhi.
NDPP chief Neiphiu Rio, a former three-time Chief Minister, was declared elected uncontested from the Northern Angami II seat. The elections in Tripura were held on February 18.