Imo, the mistake the other Republican candidates made...
They wildly overestimated the intelligence and decency of the the typical American conservative. They couldn't bring themselves to believe that the people they claimed to represent were so utterly and completely anti-intellectual, so mired in hatred and xenophobia, so unresponsive to reason, so unacquainted with humanity.
And the Dems biggest mistake--again, my opinion--was nominating Hillary. Not that she was inherently a poor candidate, far from it, but because everyone *knew* she'd be the nominee. Conservatives had eight years to work themselves into a psychotic frenzy over her candidacy, and they didn't waste a minute.