The Uddhav Thackeray-led party, an ally of the BJP in Maharashtra and the Centre, has no legislator in Goa. It had contested the last Assembly election in alliance with the Goa Surksha Manch and MGP.
"Unfortunately, the Congress has failed to act as a formidable opposition. There is a government in the state but there is no opposition.
We will be functioning as the main opposition party in Goa," Sena leader and Rajya Sabha member Sanjay Raut told reporters in Panaji.
"There is a strong resentment against the current government, but no one dares to speak out," Raut said.
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Opposition leaders are afraid of speaking against the Manohar Parrikar government as they are threatened with "consequences" if they do so, Raut alleged.
The Sena plans to expand its base and reach every village in the coastal state, he said.
The people of Goa have "accepted" the Sena and the party will build on it, he said.