Trump and his fans still believe women not tough enough to be the president
In the recent US presidential debate, Donald Trump said (albeit through gritted teeth) that the thing he most admired about his opponent, Hillary Clinton, was that she never gives up. She's got stamina. She's on the cusp of succeeding at the one thing she has wanted for decades, and she's not going to quit at the final hurdle.
Determination of this order, an inner grit and resilience, is admired by many Americans, fetishised by the myth of the 'American Dream'. If you want something enough, and you work hard enough, it can be yours - even, or especially, the presidency.
This is exactly the sort of characteristic that might, in the 11th hour, push a swing voter in Hillary's direction. Trump knows that and, having openly highlighted her strengths, he needed to do something to compensate.