Some still have that mental block, but a majority of American's are more open to the idea of female leaders. Just look at the number of women running big companies these days.
It's still a very small percentage of the total.
I think there is a deep-seated attitude among both men and women that when the going really gets tough, you want a man in charge.
Women play into this by emphasizing feminine values that sound great when there are no overriding pressing issues, but that are at best irrelevant when there is a real threat to the public.
There are a small number of women who can overcome this, but very few even try. Most go the other way. Dianne Feinstein's embarrassing, Oprah-ish questioning of John Roberts is a perfect example.