Rome operated a complicated system of government. It was not governed by a written constitution. Indeed, there does not appear to have been a moment in which the constitution came into being as system. Instead, it developed over time and its various elements found ways to work together. At least some of the modern conceptions of government (executive power, sovereignty, separation of powers between arms of government) are very difficult to map on to the Roman system.
Any description of the Roman ‘constitution’ risks misrepresenting how Roman was governed. That is because how Rome was governed was a matter of dispute and interpretation. It was continually changing over the years.
Rome was governed by its citizens, the populus. Roman citizens were defined by descent rather than residence. They were those descended from Roman citizens. People could become Roman citizens either by a legal grant of citizenship from a magistrate or by law or through the process of manumission by which a slave became freed. Roman citizenship was extended to the peoples of peninsular Italy (not Sicily) after 86 BC. There were further extensions in what is now Northern Italy under Julius Caesar.
Roman citizens were protected by Roman law and their identity as citizens was defined by law. In modern states, there is an association between citizenship and nationality. Many modern states make prospective citizens go through a text to assert their suitability for citizenship.
For Romans, the key element was not cultural (and certainly not racial), but whether they had had citizenship. Certain cultural values came to be associated with Roman citizens, such as the ability to speak Latin and the wearing of a toga) but these were not the primary characteristics.
Male citizens had four elements to their names: a praenomen (eg Gaius/Lucius/Sextus), a filiation (eg son of Gaius?Lucius/Sextus), a gentilicium (eg Julius, Claudius, Sempronius, Tullius), and a cognomen (eg Caesar, Cicero, Murena, Scipio). Sometime extra cognomina were added to names. All of those names were passed down the family line. Brothers were distinguished by different praenomina. We tend to recognise an individual by his cognomen. But Romans used their various different names.
The power of the citizens was enacted through assemblies of the people. The assemblies elected magistrates and passed laws. The interests of the people were supposedly represented by ten special magistrates, the tribunes of the plebs. The Republic (Res Publica) was what belonged to the people of Rome. There was thus a close identification between citizens and their republic.
The will of the citizens was manifested in assemblies. The two most important assemblies were the comitia tributa and the comitia centuriata. These arranged the people into different types of voting units. But both assemblies could make known the will of the people and it was the people that led to the election of magistrates and the passing of laws.
A further assembly was the Roman senate. This is often thought of, inaccurately, as the ruling council of Rome. But although it accumulate more rights and powers under Tiberius and subsequent emperors, it was an advisory council. Its main role was to advise the magistrates.
The power of the senate rested on its prestige and authority. That prestige depended on the men who sat in the senate. Although technically those who sat in the senate were defined as those who were listed as senators, in practice the senate was made up of former magistrates. Senators could be removed from the senatorial list during a census, but in normal circumstances a senator had a seat in the senate for his lifetime.
Since magistrates were those who led Rome and its people, the senate would be composed of current and former magistrates. These would be the men who had led Roman armies, governed provinces, served as priests, served as judges, administered the city, and maintained order. They had a wealth of experience.
Senators were also likely wealthy individuals.
As far as we can tell, the senate was drawn from a relatively small proportion of the Italian landed wealthy. Quite often, sons of senators followed their fathers into the senate. Many of the leading senators could trace their families back through generations of senators. They conceived of themselves as belonging to a group that had led Rome for generations, or even centuries. They were socially exclusive.
The day-to-day governance of Rome was conducted by magistrates. The senior magistrate was a consul. There were (nearly) always two consuls who came into office on January 1st and served for the year. They had very considerable power (imperium) that enabled them to implement law and to raise armies. Their power was limited by the practice of taking the advice of the senators and the laws that protected the Roman people.
Roman government tended to operate mainly through executive action: the magistrates would issue instructions in various forms. They might also act through administrative systems. The disapproval of the behaviour of a Roman citizen could lead to a censor changing the status of an individual. The Roman government also exercised moral authority: the senate, for instance, could advise and those who were advised to undertake a course of action were under considerable pressure to obey. These different systems of ensuring that the government’s wishes were obeyed meant that there was little need to invent law. Whereas most modern governmental operate primarily through a legislative process, the Romans did not.
Government also placed considerable emphasis on tradition. Those traditions added to the authority of magistrates and the senators. The traditions also honoured the status of citizens as equals. The political traditions of Rome were described in the language of morality. Very often when the Romans talk about morality, they are talking about the modes by which they interact politically with each other in public. Private morality was of much less concern.
The political system was complex and fluid. It placed considerable emphasis on consensus and concord. It was also remarkably successful. By the time of the assassination of Caesar, the system of government had, with some modifications been in place for more the 450 years.