Warning: mouthful of post
I stumbled on this book randomly - doing research on the space program via Google books.
It is an excellent account of Gorbachev ascension, how it happened, when, and what obstacles he had to level to get there in March 1985.
I learned a lot of things - notably how Brezhnev literally sabotaged its own succession until way too late, more or less dooming USSR in the process !
If I understood the book correctly - by the end of the 70's Breznhev more or less eliminated any potential rival in the close political circle around him.
Andropov and Gorbachev were reformists, but they were also dark horses that in "ordinary" circumstances would have never reached the summit.
Even in the USSR, a former head of KGB like Andropov could never reach the Kremlin.
It was Breznhev never ending agony that made this possible.
Chernenko was a more obvious pretender - but he was ailing.
Suslov was another pretender, but he died even before Breznhev himself.
Romanov was Gorbachev most dangerous rival, but (fortunately) he was doomed from day 1 a) by its name b) by his own siliness - he was dumb as hell, not politically astute at all. Gorbachev outsmarted him anyday.
From the book, one can say that Gorbachev path to power was a mix of astuteness and luck - lot of luck. Lot of people died at the right moment, clearing the path.