I think most of the backing came due to two facts:
(1) the USSR needed reforms, that was obvious.
Still, they probably wanted a long term leader that could be influenced => a younger one, after the two short reigns of Andropov and what-was-his-name… Probably, the influentials wanted some change, but surely not that much that it would threaten their power-base, one of the reasons for the coup that failed due to Jelzins help for Gorbachev.
(2) The general secretary and all the other positions he fills made him the most powerful man in the USSR. Probably, whoever put Gorbachev into place, underestimated him. Pretty much the same underestimation that Hitler faced when he was appointed Chancellor, but luckily Gorbachev used that to reform positively, and i consider the new freedom of information, the right to ask wether the party is right and that the crucial reform.
So, I don’t think that there was “real” backing, i believe Gorbachev was kind of a compromise candidate, kind of a makeshift. Of course, once he was into power, you ccouldn’t remove instantly, that would have meant that the party made a mistake (which was not “allowed” before Gorbachev). And he was clever enough to stay into power, and use his popularity at the beginning (which soon, after the economy started to collapse openly, was reduced to more or less nothing) to enforece the reforms.
