It's easy to poll high support numbers for things like "border security" if you avoid the details of what that means. For decades, border security did not mean explicitly banning the entry of all people coming from certain countries based on a campaign promise of religious-based discrimination.

I'm guessing that if you polled the approval of "ban all Muslims, including legal permanent residents", you may end up with lower numbers.

EDIT: this doesn't even mention that, at a certain point, it's necessary to draw the line at fundamental rights that cannot be voted away. Thankfully we have organizations such as the ACLU who fight to protect that line.