The press and the Democrats hated Nixon with a passion it’s hard to fathom. I would state in response, if they thought there was any chance whatsoever to have prevailed, they would have challenged the pardon at some point when a Democrat held the office.
Having driven him from office and totally humiliated Nixon, I think it might have generated a backlash if they pursued him. I think the pardon was a way of closing the chapter and “moving on.”
Nixon was “punished” for Watergate. Hillary, on the other hand has so far escaped punishment for her crimes. If Obama pardons her, I would certainly support a challenge to the legality of the pardon.