You don't want JUST protein. You need to eat something really carby and throw some protein in with the mix. Carbohydrate=energy. Your body is about to go through the workout of your life. You need energy. If you eat just lots of carbs, your body uses them up really quickly though. If you pair them with a good protein source, it helps stabilize your blood sugar and lets your body use that energy more efficiently. You should carb load the night before, like a giant spaghetti and meatballs dinner sort of thing, and then for breakfast, maybe eggs and pancakes and some fruit?
You need adequate protein as well, it should not be just an afterthought. Without protein you cannot fully utilize the carb energy you have. Without enough protein, you also face a higher risk of dehydration.
I agree w/ Ghostmonkey. Also, that's not exactly how you carb-load. I would NOT actually carb-load late in pregnancy, because it first involves a prolonged starvation of carbs, which tricks your body into storing that massive carb intake as glycogen, which your muscles slow-burn, giving the endurance. Without being tricked, your body doesn't do that, and you just have a whole bunch of fast carbs on board. So GM is right.
It is true that glycogen stores get low in laboring women, and we shake, and we feel that we hit rock bottom in the hardest hour of transition. But I don't think a certain amount carbs is the answer. It just is. That's the big physical and mental challenge that we try to prepare ourselves for. Then adrenaline helps us through the rest.
Try to eat a balanced, protein-rich meal that will sustain you for the long haul, as you might not have a chance to eat for quite some time.