You would need a POD in late 1986 or early 1987. The battle between the hardliners and the reformers was more-or-less resolved with the sacking of Hu Yaobang as Secretary General. If Deng manages to ensure the survival of Hu, then there is potential that the reformers will be in a stronger position when Hu dies in 1989 (his death is what started the protests in the Square) and that stronger position is inherited by Zhao Ziyang.