Neither Presidents GW nor GHW Bush will not be campaigning for Trump. The closest thing the Republicans will have to a rock star is Chris Christie or maybe Paul Ryan.
Meanwhile, Hillary will have President Obama, President Clinton, Michelle Obama, Elizabeth Warren, and Bernie Sanders, who like them or not, are capable who are attracting massive crowds. I'm guessing that even Vice President Biden and Senator Al Franken are bigger draws than Christie or Ryan.
Endorsements are bullshit, but big crowds convert into volunteers, voter registrations, and money. Just ask Bernie.