Congress and NCP leaders had announced their alliance for Lok Sabha in Goa earlier this afternoon, just after the news of Muthalik joining BJP came.
They said it was one of the reasons that they decided to form an alliance.
"The alliance happened because NCP realised that we should fight against BJP and not amongst ourselves," Congress' South Goa candidate Aleix Reginald Lourenco told reporters, in the presence of state unit presidents of Congress and NCP.
Muthalik, the controversial chief of Sri Rama Sene, is linked with the attack on women at a pub in Mangalore in 2009.
The previous Congress-NCP regime in Goa had disallowed Sri Ram Sene to start a branch in Goa.
"Muthalik had vowed that he will join BJP and enter Goa," NCP Goa unit's chief spokesman Trojano D'Mello said, claiming that if his organisation comes to the coastal state it would harm the local social fabric.