Under the monarchy, there was an elective king, who was the commander of the army, the chief priest, and the supreme judge.
The king was replaced by 2 Consuls elected by the Assembly of the Centuries ( the army ). At first, the Consuls were both chosen from the patricians. The consuls served for only a year, then entered the Senate for life. Each consul culd veto the action of the other. The Consuls were endowed with the ex-king's imperium. They led the army, served as judges, and had religious duties. While the consuls had absolute power of life and death on a campaign, within Rome the citizens had the right to appeal to the popular assembly all cases involving capital punishment. From 233 to 133 BC only 26 familites provided 80 % of the consuls and only 10 of these families accounted for almost 50 %. The patricians dominated the consulships.
The Senate was a deliberative body that had control of finances and foreign policy. Its members were predominatly patricians. There were about 300 members who served for life. The Senate was a narrow oligarchy of patricians.
At first, only patricians could sit in the Senate, be consuls, and hold the important public offices. They tried to prohibit marriage outside their group. After the "STRUGGLE OF THE ORDERS", the plebians succeeded in counter-balancing patrician power in the Assembly of the Tribes and the Tribunes. They succeeded in opening the patriciate to marriage with outsiders as well as the priesthood and one of the consuls. The need for plebian service in the army as well as the withdrawl of the plebians, when angered by patrician resistance to their demands, led to "democratization.
The plebians established their power in the Assembly of the Tribes and the Tribunes. In 494, the plebians withdrew from Rome, formed an Assembly of the Tribes and elected tribunes to protect the arbitrary power of the magistrates. They declared the TRIBUNE SACRED AND INVIOLATE. ANYONE KILLED A TRIBUNE COULD BE EXECUTED WITHOUT TRIAL. THE TRIBUNES COULD VETO THE ACTS OF CONSULS AND OTHER MAGISTRATES. TRIBUNES WERE ELECTED FOR A YEAR.
IN 287 BC, THE ASSEMBLY OF THE TRIBES GAINED THE RIGHT TO PASS LAWS THAT WERE BINDING ON ALL ROMAN CITIZENS WITHOUT FURTHER ACTION BY THE SENATE. THE ASSEMBLY APPROVED OR REJECTED TREATIES AND ALLIANCES. IT VOTED WAR AND PEACE. THE ASSEMBLY WAS THE "DEMOCRATIC" ELEMENT IN THE ROMAN CONSTITUTION.