Patkar's name is in the list of 20 candidates announced by AAP for the general elections today.
NCP sources said sitting MP Sanjay Patil, who is also the president of the Sharad Pawar-led party's city unit, was facing resentment from inside the party and from Congress.
"Congress accused him of sabotaging its candidates in the municipal elections two years ago. NCP corporators and senior leaders are also not in favour of him," a source said.
Patkar, who shot to fame with Narmada Bachao Andolan, has some base in the constituency as her National Alliance of Peoples Movement (NAPM) works in the slums of Mankhurd.
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"We had done well in this area in the last Lok Sabha polls, but Patkar will eat into our votes. Ghatkopar East and West as well as Mulund have Gujarati population, and BJP is likely to do well there given the Narendra Modi factor," said an NCP leader.
NCP sources said the party had already realised that it could be difficult to retain this seat, but Patkar's entry had made it difficult further.