All elections shall be by ballot. Every citizen of the United States, of the age of 18 years or upwards, who is a resident of the State as of the time for the closing of registration next preceding the election, shall be entitled to vote in the ward or election district in which he resides at all elections to be held in this State. A person once entitled to vote in any election district, shall be entitled to vote there until he shall have acquired a residence in another election district or ward in this State.