Patkar's name is in the list of 20 candidates announced by AAP for the general elections on Sunday.
NCP sources said that 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.
The constituency has a sizable Dalit population, which had helped Republican Party of India (RPI) leader Ramdas Athawale in the past. But now Athawale, recently elected to Rajya Sabha, is with Shiv Sena-BJP alliance.
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.
"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.
Former MP and BJP leader Kirit Somaiya could be the Sena-BJP alliance's candidate from the constituency.
NCP sources said that the party had already realised that it could be difficult to retain this seat, but Patkar's entry had made it difficult further.

Of the six Lok Sabha seats in Mumbai, last time Congress won five seats and NCP won one seat.
Congress sources said that it is too early to say whether Patkar would eat into Congress-NCP's votes or BJP's votes.
State Congress spokesman Anant Gadgil said that Raj Thackeray's MNS and AAP had disappointed the middle-class which was looking for an alternative to BJP and Congress.
"AAP was given a chance to govern in Delhi, but they have not been successful. AAP and MNS in their initial stages were media creations. In politics, such creations do not last," he said.
Congress-NCP leaders felt the other AAP candidates namely Meera Sanyal (Mumbai South, against Union minister Milind Deora), Mayank Gandhi (Mumbai North West, against AICC General Secretary and CWC member Gurudas Kamat), Vijay Pandhare (Nashik), Subhash Ware (Pune) and Anjali Damania (Nagpur, against BJP leader Nitin Gadkari) will not have much
"AAP has no leadership in the state and will not make a difference. Sanyal had unsuccessfully contested against Deora as Independent candidate last time, while Damania's presence in the fray might help sitting Congress MP Vilas Muttemwar to retain Nagpur for the eight consecutive time," a source added.


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