Tanga Byaling won from Nacho Assembly seat in Upper Subansiri district of the BJP-ruled state for five consecutive terms.
Former Konsa West MLA Yumsen Mate and six other political leaders also joined the Congress.
Lok Sabha and Assembly polls will be held in the state on April 11.
He said the Congress would consider poll alliance in a few seats with parties like the Janata Dal (Secular) and People's Party of Arunachal.
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