In the early days of the Republic (soon after its founding in 509 B.C.), there was significant social conflict between the aristocratic class (Patricians) and the poor (Plebians). At that time, the entire government apparatus was controlled by the wealthy and the lower class had no rights. Unhappy with their situation, the Plebian class began to protest an indiscriminant application of law that saw them assaulted or dragged from their homes without the ability to defend themselves. After a protest in 494 B.C, the Patricians agreed to create a new magistrate who could protect the people from abuses. Originally two Tribunes were elected, then five, and finally, in 457 B.C, their number was expanded to ten.
Tribunes were designated as inviolate; meaning they could not be touched or harmed by any person. They had the right of intersessio, and could step in front of an assailant to protect a Plebian from harm. If a Tribune were injured in any way, the offender would become an outlaw and forfeit his property to the state.
During the year of their term of office, Tribunes were required to leave the doors of their homes open day and night in case someone needed help. If they left the city, they could only be absent for twenty-four hours.
This personal protector role expanded over time and Tribunes eventually acquired a powerful role in government. They were given the power to block legislation, call meetings of the Senate, or compel Consuls to comply with decrees of the Senate. In 286 B.C, the law Lex Hortensia was passed which gave the Plebian class, through its own assembly, the right to pass laws binding on all of Rome. Tribunes were elected by the Plebian Assembly and presided over its meetings.
But the aristocratic class came to regret what they had done in giving the lower class so much power, so they sought ways to influence the behavior of Tribunes on their behalf. By pressure or bribery, the rich began to induce the Tribunes to veto legislation they were opposed to. This problem came to a head during the Tribune of Tiberius Gracchus (133 B.C.) when he removed another Tribune from office for vetoing a land bill favored by the people but opposed by the Senate. Tiberius was later assassinated by the Senate for trying to accumulate power. This single event incited the Plebian class against the Senate, beginning the process of the decline of the Senate as an institution and the end of the Republic.