LBJ must have still had support of a large majority of working class Democrats, particularly those over 35 or so. Those who later supported HHH in the primary for sure, and most of the blue-collar element of RFK's multiracial coalition. LBJ most likely would have won re-nomination had he sought it, but he knew he would have problems in the general.
Nobody really supported HHH in the primary (other then a handful of votes for his “favorite son” stand ins who always finished a distant third) which was part of what led to the reforms after that election. The primary process was nakedly undemocratic at the time.