The Roman historians agree that the origins of the Roman Senate go way behind the establishment of the republic and that this body played a major role in politics even in the times of early Roman kings. Romulus, the founder of the city and its first king, is usually attributed with the act of its establishment.
The original number of senators was set to the round 100. The process of nomination was closely connected to another innovation of the first king – the establishment of tribes and curiae. Romulus divided the Roman nation into 3 tribes and each tribe to 10 curiae. Each of those units then gained the right to make their nominations for the senatorial posts. Each tribe, as a whole, chose three men, each curia in the tribe another three.
3 men x 3 tribes + 3 men x 30 curiae = 99 nominated senators
Where did the last one come from? Romulus himself selected one man.
According to Dionysius of Halicarnassus, Romulus created the Roman senate not just as an advisory body, but as one that would hold the real power to make decisions. After the Senate was created, it was necessary to define its powers and distinguish its areas of competence from those of the king.
The king always held a primary position in the matters of religion, the highest command in war, personally acted as a judge in the most serious cases (lesser crimes were left to senators) and had the right to summon the Senate. At the meetings, the king spoke first and let the others know his opinions, but he lacked the absolute power. Senate would vote and the majority made the decision.
Plutarch, in turn, tells us that, at least by the end of his reign, the power stayed with the king alone. The senators gather only to hear the decisions of Romulus without being actually involved in the decision making. This was then one of the reasons why they conspired against the king and assassinated him (according to one version of the story of Romulus’ death).
The count of the senators changed after the war with the Sabines (the one started by the incident known as the Rape of the Sabine women). The signed peace treaty established a legal ground for partially merging the two tribes. Many Sabines stayed in Rome, their leader Titus Tatius became a king of Rome alongside Romulus and they were given a voice in the Senate. Some say the number of senators doubled, others provide a lower estimation – increase by 50% (=50 new senators on top of the original hundred). The senators didn’t consult the kings at once, but each king met separately with “their people” and just then met each other.
With Tatius and after his death Romulus carried on his warlike tendencies and fought successfully with many neighbouring cities (Fidenae, Cameria, Veii). Those wars often ended with Rome welcoming new people from the defeated tribes. The intake of new people created unrest and Romulus was accused of discriminating them against the original Romans. This might have been the cause of his demise. The new citizens conspired against him and eventually assassinated him.
Alternative theory speaks about the patricians opposing his style of ruling, getting more and more tyrannical with time, and assassinating him in the Senate. In order to avoid chaos, the killers divided his body to many smaller parts and carried them out of the Senate building separately under their clothes to keep the deed secret for a while.
All In all, we can conclude that even though Romulus established the Senate and provided the new institution with competencies of no small scale, the Senate acted in opposition and may even be responsible for his assassination.
Dionysius of Halicarnassus – The Roman Antiquities
Plutarch – Parallel Lives