let's not forget Trump's bad record in endorsing candidates in 2018 and I did not see Bush campaigning hard for McCain. No one has a good track record in that respect which is why they dont or shouldnt do it...
Okay but we were talking about Joe and Obama. And Joe is, shall we say, an enhanced case of a candidate entering a race with a campaign strategy of promoting himself as the spiritual and political heir of a previous office holder- ie. Obama. Joe promoted the theme that he was a continuation of the Obama legacy. In such cases, it always gets dicey if it remains unclear whether the other person actually endorses you. Thus- in regard to your example- Bush may have not campaigned enthusiastically for McCain, but neither did McCain enter the race by arguing that he was the heir apparent to George Bush and promised to be another George Bush if elected. And so on in regard to other examples.