During First Man in Rome and well into The Grass Crown, it was possible to ignore the fine points of Roman governing institutions. McCullough might write about voting in the Centuriate Assembly or the Tribal Assemblies, but the exact nature of these assemblies was less important than the outcome of the vote. Votes in the Senate also carried a lot of drama and were presumed to have the force of law. There was a section in the glossary explaining all these institutions, but it was easy to skip over because the precise nature of these institutions was not the focus of the story. The institution that received the most focus was the Roman army and the changes Marius made to it. The institutions of the Roman (civil) government do become important later on in The Grass Crown because the changes Sulla makes to them are critical to the story and highly revealing as to what his ultimate vision was.
One thing is clear. By the time McCullough addresses, most of the advantages of being a patrician were gone. Most of the old patrician families had seen their wealth diminish, and the richest Romans were mostly plebeians. If a Plebeian achieved consular rank, his descendants would be considered nobility, just not quite as high nobility as a Patrician. Patrician woman were allowed less independence than plebeian women, so patrician men had some advantages in domination over their women. A handful of offices were restricted to patricians. Most important was Princeps Senatus (First Man of the Senate). While censors held their office for five years and other magistrates for one, there was no limit on how long the Princeps Senatus could hold his office, which made him a very powerful official. But it also limited any individual patrician's chance of ever reaching that office. The Senate was divided into groups of ten, with each one led by a patrician, so one senator in ten had to be a patrician. And a handful of other offices were restricted to patricians, but most were priesthoods with no real power. Weighed against this were powers restricted to plebeians that we shall address shortly.
Roman governmental institutions changed over time, but McCullough addresses the institutions as they existed before Sulla's revolt.
The Senate. Senators held their office for life. Anyone elected as magistrate above a certain rank was automatically admitted to the Senate. Other senators were chosen by the censors, who could also remove a senator for misconduct. The Senate was a powerful body. It alone authorized expenditures, appointed provincial governors, and and was in charge of administering the provinces. It also took the lead in foreign affairs and could declare emergencies in which the other branches of government were suspended. It was not, however, a legislature in the modern sense. While it is often shown voting on legislation, in the end, its vote was only advisory. Only the popular assemblies could pass binding legislation. This not only means that legislation the Senate voted for would not become law unless passed by the Senate, it also means that a popular assembly could pass legislation that the Senate had advised against or without consulting the Senate at all, although to do so was considered the mark of a demagogue. It is a mark of Drusus' resolve to work within the system that he vows never to bring a measure to the Plebeian Assembly without first consulting the Senate. After getting Senate approval for his Gracchi-like program, it is an easy matter to have it approved by the Assembly. But when the Senate rejects Italian citizenship, he resolves to take it to the Assembly anyhow. This is what leads to his assassination.*
The popular assemblies. There were three of these, the Centuriate Assembly, the Assembly of the Whole, and the Plebeian Assembly. The latter two were called the Tribal Assemblies. Legislation only had to pass one of these assemblies to become binding law. All three assemblies practiced the direct democracy of the people legislating by themselves rather than through representatives. However, none operated on the basis of one-man-one-vote in the manner of the Athenian Assembly, a New England town meeting, or a modern initiative or referendum. Rather, the people assembled were broken into sub-groups. Each sub-group voted, and the decision was made by how the majority of sub-groups voted. Since all assemblies were too large and cumbersome to be able to draft legislation, the appropriate official would call the assembly into session and introduce proposed legislation for its consideration. It is not clear to me whether the assembly could debate or amend the legislation or simply vote it up or down. McCullough implies, however, that it could debate and amend. There were also differences between the assemblies.
The Centuriate Assembly. This assembly was divided into five classes, based on property holding. A vote of the majority of classes prevailed. The very poorest Romans, who had no property, were not allowed to vote in the Centuriate Assembly. The Centuriate Assembly elected Rome's most important executive officials, the consuls, praetors, and censors. Consuls were Rome's chief executives, two at once, elected to one-year terms. Consuls were supreme military commanders and also the top civil official. If one consul was in the field, the other administered Rome. Praetors were secondary executive officials, serving as judges and handling much of the day-to-day government. When both consuls were in the field, the urban praetor became chief civil executive. The censors, uniquely among elective officials, served five-year terms. They conducted the census, chose or deposed Senators, and oversaw public morals. They lacked the power to impose criminal penalties, but could impose political penalties (i.e., removal from office, being barred from office, or disenfranchisement). The Centuriate Assembly could try major cases or pass legislation, but as a practical matter it was too cumbersome to do so.**
The Assembly of the Whole. This assembly was divided into "tribes." "Tribes" were not ethnic or familial groupings (as that term is used today), but (at least originally) geographic groupings. There were thirty-five tribes. A vote by 18 of the 35 tribes prevailed. The Assembly of the Whole passed legislation and elected a variety of lesser executive officials who did much of the day-to-day management of government, but whose jobs were mostly administrative with little or no policy-making authority. The consuls presided over the Assembly of the Whole, called it into session, and proposed legislation to it.
A few comments are in order about the tribal nature of the Assembly. McCullough emphasizes that because urban Rome had only four tribes, it could always be outvoted by the 31 rural tribes, unfairly giving excessive weight to rural interests. It should be noted that the Athenian Assembly, operating on the basis of one-man-one-vote had the opposite problem. Because traveling to Athens to attend the Assembly was a burden on rural residents (rich or poor), the Assembly was unfairly weighted towards urban interests. Although McCullough does not emphasize this, presumably in the "tribes" farther from Rome, only the richer and more eminent citizens could afford to make the journey. This would not necessarily be unwelcome to ordinary residents. Presumably Romans defined their interests vertically (by geography) as well as horizontally (by class) and in many areas locally important issues would unite all citizens, rich and poor. Ordinary citizens would presumably be happy to see their wealthier neighbors go to Rome to uphold their interests, guaranteed weight above the number of people attending by the tribal system. This could serve as a sort of primitive form of representation, But it was a system of representation suffused with rotten boroughs, i.e., districts with weight far in excess of their population.
Citizens too poor to vote in the Centuriate Assembly did vote in the tribal assemblies. However, poorer rural residents could not afford to go to Rome, while poor urbanites were all lumped into one of the four urban tribes that were outnumbered by the 31 rural tribes. On the other hand, when a Roman moved from one area to another, his tribe did not change. Presumably this allowed many urban residents to vote with their rural tribes. Given the added advantage that attending the Assembly was easier for urban than rural dwellers, this would have destroyed any sort of coherence for tribes as political units, but also lessened the unfair domination of rural interests. Counterbalancing this, all freedmen were made members of one of the four urban tribes.
Plebeian Assembly. The Plebeian Assembly was divided into the same tribes as the Assembly of the Whole, so all comments on tribes apply equally to the Plebeian Assembly. It differed from the Assembly of the Whole in that Patricians were not allowed to attend, or even to be present when it was in session. The Plebeian Assembly had the same powers of legislation as the other Assemblies. It elected ten Tribunes of the Plebs. Any one Tribune could call the Plebeian Assembly into session, preside over it, or propose legislation to it, with or without consulting the Senate, although failure to consult with the Senate was considered the mark of a demagogue. Tribunes had no executive powers. What they famously did have was the veto, not only over legislation, but over any aspect of government. And there were ten of them, each with the same veto! Thus any advantages in being a patrician had to be weighed against the disadvantages -- ineligibility to participate in the Plebeian Assembly which had the power to pass binding legislation (indeed, it ultimately became Rome's primary legislature), and ineligibility to be elected Tribune. A patrician was cut off from two important sources of political power.
This lesson in Roman civics will become important when Sulla comes to power and makes major changes in these institutions.
*Caesar will later do the same thing.
**Wikipedia has a much more complex description, emphasizing the Centuriate Assembly's military and oligarchic features.