He is trying to be like Caesar (obvious parallel btw). With victory in the field, he will have the backing of the common people for being a returning hero, and (hopefully) a great deal of respect with the army because they know firsthand that he is a capable leader. With those two groups backing him, he could overthrow the senate and possibly the aristocracy.

However, I wouldn't count on the military backing him after the tremendous losses he is incurring against Avalon. This is one of the reasons the senate is shaming him, and it is especially bad because they are "uncultured barbarians" who have neither the percieved organization nor technological advantage when it comes to war machines. I guess most aren't considering magic or the fact that many of the living elves have sided with or sent representatives to Arawn.