Save Our Building Societies is calling for job-protection guarantees for Birmingham Midshires' employees ahead of the society's proposed £750m merger with Halifax.
The pressure group is urging Birmingham Midshires members to push for a 10-year safeguard for jobs and branches. Halifax says it "intends to avoid compulsory redundancies".
Sobs coordinator Bob Goodall says: "The Halifax will slash and burn. They have only promised no compulsory redundancies for three years."
Members finished voting on Tuesday and an EGM is to be held this Friday.