The Constitution of the Roman Empire was an unwritten set of guidelines and principles passed down mainly through precedent. After the fall of the Roman Republic, the constitutional balance of power shifted from the Roman Senate to the Roman Emperor. Beginning with the first emperor, Augustus, the emperor and the Senate were theoretically two co-equal branches of government. In practice, however, the actual authority of the imperial Senate was negligible, as the emperor held the true power of the state. During the reign of the second emperor, Tiberius, many of the powers that had been held by the Roman assemblies were transferred to the Senate.
The powers of an emperor existed by virtue of his legal standing. The two most significant components to an emperor's power were the "tribunician powers" and the proconsular imperium, or the power to command. The tribunician powers gave the emperor authority over Rome itself and the civil government, while the proconsular powers gave him authority over the provinces and the army. While these distinctions were clearly defined during the early empire, eventually they were lost, and the emperor's powers became less constitutional and more monarchical. The traditional magistracies that survived the fall of the Republic were the Consulship, Praetorship, Plebeian Tribunate, Aedileship, Quaestorship, and Military Tribunate. Any individual of the senatorial class could run for one of these offices. If an individual was not of the senatorial class, he could run for one of these offices if he was allowed to run by the emperor, or otherwise, he could be appointed to one of these offices by the emperor.
Imperial constitutional history
The general who won the last civil war of the Roman Republic, Gaius Octavian, became the master of the state. In the years after 30 BC, Octavian set out to reform the Roman constitution. The ultimate consequence of these reforms was the abolition of the republic and the founding of the Roman Empire. When Octavian deposed his fellow triumvir, Mark Antony, in 32 BC, he resigned his position as triumvir, but was probably vested with powers similar to those that he had given up. Octavian wanted to solidify his status as master of the state whilst avoiding assassination.
On 13 January 27 BC, Octavian gave up his extraordinary powers, including his proconsular imperium, triumvirate authority, and powers granted in the consensus universorum, back to the Senate and people of Rome. However, he kept the Senate's grant of tribunician power in 36 BC and remained consul for 27 BC. He received from the Senate, doubtlessly at his own suggestion, greater proconsular imperium for a period of ten years, giving him supreme authority over the provinces, and therefore, the entire Roman military. These various powers and offices both allowed him to maintain his legal authority over the empire at large and claim that the Republic was restored. The wide-reaching grant of proconsular authority was precedented in lex Gabinia'''s grant of similar authority to Pompey in 67 BC. Furthermore, the proconsular powers were theoretically outranked by the consular powers held by the sitting consuls, allowing Octavian to claim he did not stand above any other magistrates of the state. To maintain his control in Italy, which was not considered a province, Octavian had the Assemblies elect him to the position of consul.
At this point, the Senate also granted Octavian the title "augustus" and the position of princeps senatus, or the first Senator. When Augustus, as Octavian was renamed, gave up the consulship in 23 BC, the Senate granted him an expansion of his proconsular authority, with legal authority at the same level as those of the normal consuls. Furthermore, Augustus expanded the use of the tribunician powers granted in 36 BC, which allowed him to interfere in administration, convoke the people, propose legislation, veto other political bodies, etc. After these final reforms had been instituted, Augustus never again altered his constitution, although he did periodically assume the office of censor to affect changes in the Senatorial rolls (alone in 8 BC, and with Tiberius in AD 14) and hold the consulship (twice after 23 BC). When, in 20 BC, a famine in Rome led to the people offering Augustus the dictatorship, he refused, instead taking on the cura annonae: the administration of the grain supply. And when the ten-year term of his proconsular authority elapsed, he continued the fiction of the temporary nature of his authority, having it renewed in 18 BC, 13 BC, 8 BC, 3 BC, and 8 AD. While there were minor changes in the provinces which he governed by nature of his proconsular authority (Gallia Narbonensis and Cyprus were returned to the Senate's administration in 22 BC), its overriding nature did not change until after his death.
Augustus' final goal was to figure out a method to ensure an orderly succession, something necessary in any monarchical constitution and to avoid the resurgence of civil war. Augustus could not transfer his powers to a successor upon his death, as they were given specifically to him for some fixed term or during his life. Thus, any successor would need to have his own authority and influence. In 6 BC Augustus granted tribunician powers to his stepson Tiberius and recognised Tiberius as his heir. In AD 13, a law was passed which made Tiberius' legal powers equivalent to and independent from those of Augustus.
On Augustus' death, Tiberius had to take control of the state, since the nominally republican constitution did not allow for the dead emperor to bequeath powers. With the support of the Praetorian Guard and the army, he quickly took control of the state. During his reign, the power to elect magistrates was transferred from the assemblies to the Senate and he ended the practice of proposing laws before them. When Tiberius died, Caligula was proclaimed emperor by the Senate. In 41, Caligula was assassinated, and for two days following his assassination, the Senate debated whether to restore the Republic. Due to the demands of the army, however, Claudius was ultimately declared emperor. While Claudius' antiquarian interests resulted in his attempts to revive the old censorship, these powers became increasingly folded into imperial prerogatives.
Over time, the consulship was increasingly devalued, as the practice of selecting suffect consuls expanded, with sometimes up to 12 pairs of colleagues being elected every year by the Antonine period. With its duties of state increasingly being folded into the office of emperor, its judicial authority was emphasised in the Senate's new role as a criminal court. The consuls lost practically all their political powers and took over responsibility for organising public games to celebrate holidays and imperial events. The political power of many of the Republic's other magistracies was also neutered and high offices of state effectively became municipal offices with primarily administrative duties only in Rome. The praetors became administrators of the grain dole and games, retaining some judicial authority over civil and criminal cases until the third century. The duties of the aediles were to organise policing of games and public funerals, their judicial authority stripped away in 36 AD, and the office fell out of use by the middle of the third century. The tribunes, who theoretically retained their veto powers, which were irrelevant because they could be overridden by the emperor, became presidents of various new city regions.
Vespasian's reign saw the reorganisation of the Senate from a body of aristocratic Romans to one of the Empire's aristocracy, with its membership and privileges given by the emperor. Domitian's reign marked a significant turning point on the road to monarchy and the end of the constitutional arrangement whereby the Senate and Emperor ruled the Empire together. After making himself consul for ten years, Domitian made himself Censor for life, and unlike his father, he used these powers to further subjugate the Senate by controlling its membership. Throughout his reign, he abrogated the rights of senators, only referred to it the most trivial matters, prevented it from acting on matters of any import, and forced it into compliance through his use of censorial powers. In effect, it became a rubber stamp. While the Senate regained some authority when it elected the emperor Nerva, by this point, the powers that were theoretically held by the last remaining republican institution of any import, were clearly defined only by the will of the emperor.
Senate
After the fall of the Roman Republic, the constitutional balance of power shifted from the Roman Senate to the Roman Emperor. Beginning with the first emperor, Augustus, the emperor and the senate were technically two co-equal branches of government. In practice, however the actual authority of the imperial senate was negligible, as the emperor held the true power of the state. As such, membership in the senate became sought after by individuals seeking prestige and social standing, rather than actual authority. During the reigns of the first emperors, legislative, judicial, and electoral powers were all transferred from the Roman assemblies to the senate. However, since the control that the emperor held over the senate was absolute, the senate acted as a vehicle through which the emperor exercised his autocratic powers.
The first emperor, Augustus, inherited a senate whose membership had been increased to 900 senators by his predecessor, the Roman Dictator Julius Caesar. Augustus reduced the size of the senate to 600 members, and after this point, the size of the senate was never again drastically altered. One could become a senator by being elected Quaestor (a magistrate with financial duties). However, one could only stand for election to the Quaestorship if one was of senatorial rank, and to be of senatorial rank, one had to be the son of a senator. If an individual was not of senatorial rank, there were two ways for that individual to become a senator. Under the first method, the emperor granted that individual the authority to stand for election to the Quaestorship, while under the second method, the emperor appointed that individual to the senate.