Attacking Air Force One is probably the dream of many of America's enemies. It would also be a significant mistake. This is a plane that's more than capable of holding its own when under attack.
We've already covered the flares under the wings, which can scramble the senses of heat-seeking missiles when launched. Air Force One is also equipped with high-tech electronic radar jamming equipment, which would confuse enemies even further. Radioactive fallout from a nuclear attack can't even get through the hull.