Winston says that the Party will ultimately fail. What does he say will destroy the Party ?
I'm not sure exactly what you're referring to, but I think you're referring to Winston's "faith" in the Proles. This faith is based on some Romantic and idealistic view that the dis/unorganized Proles coudl somehow realize how miserable their situation is and rise up in revolt to overthrow the Party.
This is total nonsense. O'Brien makes it clear that what makes the Party different from all that has come before is its ability to use power as power ... not as a means to reach some "good" end, but rather as an end in itself. According to O'Brien, past dictatorships have made the mistake of not admiting that their final end was power.
This isn't something the Proles can deal with; no one in their society can deal with it.