Clinton isn't underestimating Trump though. She and other democratic groups have been running non-stop oppo research, focus groups, and is already reserving huge ad buys in the swing states that will force Trump into worse and more expensive time slots.
The rest of the Republican field decided to be cucks and wait until it was too late to hit back against Trump. She's going to hit him back twice as hard for every blow he throws at her.
This is the conventional campaign memo stuff It didn't work. Trump isn't running as the boss everybody loves hes running as the asshole boss that gets things done. Look at all the money they wasted fighting him in the primaries with pretty much everything. Trump is going to pound jobs jobs jobs NAFTA.. While Hillary attacks eventhough her negatives are just as high.
The general election audience is vastly different than the predominantly white Pub electorate. Trump was toxic to the GOP field and was able to prevail due to a rather fortuitous set of circumstances (divided field, weak field, WTA primaries), and his general election favorables are absolutely toxic; something like 30/70 fav/unfav, which are drastically worse than Secretary Clinton's.