Good point, but he really didn't win running on anything other than a vague and ever-morphing "change."
When he got tripped on the details -- whether past associations, questionable political contributions, his abortion stand in the Illinois senate or other issues that contradicted the official vagueness -- he fought back by labeling the charges "distractions," and "smears."
He's doing basically the same now, wanting no discussion of details, labeling his opponents as obstructionists, and couching his argument in generalities and fear-mongering.
The media likes the idea of a public opinion “mandate” because they influence public opinion. But in a Republic we elect leaders to lead, not to follow malleable public opinion.