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.