Pratic, they will be roaming all around the campground in the dark when everyone is asleep. You cannot keep them away. They stealthily check for smells of food -- bears' noses are about a thousand times more sensitive than humans.
If you are camping overnight at the portal, use the bear lockers, and make SURE they are properly latched. They only want your food, they never bother humans, (...unless they are being challenged once they have possession of some food. I know, I did that once.)
If you are getting up early to day hike, do not put food in your day pack and leave it for a minute on the picnic table. When it is dark, the Whitney Portal bears will sneak up, grab the pack and run.
The bear problem is probably worse at Whitney Portal than in Yosemite due to the pre-dawn day hikers with food in their day packs.
Here's the thread with the bear video from last year.
Bears at Whitney Portal It is pretty crazy that the woman recorded the bear for 3:15 before she figured out how to scare it away.
