If you loose the popular vote, whether fairly or fouly, it become harder to argue for a mandate.
“...it become harder to argue for a mandate.”
The Constitution does not require a mandate; only enough electoral votes. Popular vote is not the main concept in determining the outcome; the electoral college is.
However, do you really believe that Trump will abandon his platform and rely on consulting the ones who tried to destroy him? Why should the vanquished party be co-equal to the victorious party?
A “mandate” is always the cry of the losing party.