“This access key has what looks like the presidential seal on it. It appears as though it's used to gain Presidential level access to computers.”
The Presidential access key is a quest item in Fallout 2.
This key can be used on the terminal in the Oil Rig foyer to activate the counter-insurgency protocols, causing the foyer turrets to fire on Frank Horrigan and Granite's Control Company.
Carried by President Richardson. It cannot be stolen from him, it will have to be taken from his corpse.