According to me there is nothing like the compact majority. There are just a group of individuals with individual opinions that manage to conform to many people's opinions.
At the begining of Act 4 there is a discussion between citizens that reveals that they blindly follow their leader's opinions. From act five the mayor, the land lord, glaizer, Mrs busk, Mr Vic, Aslasken and Morten kill refuse to help Dr Stockman and up set his plans all due to public opinion. All of them however are public leaders in their own right. Therefore Dr Stockman initially had the compact majority behind him but he made the mistake of insulting the crowd which made the town leaders change their stand to conform to public opinion. So he was not mistaken but was arogant to the change of public opinion and the compact majority by extension.