After the expulsion of the last King Lucius Tarquinius, most of the powers and authority of the king were given to the newly instituted consulship. Originally, consuls were called praetors, referring to their duties as the chief military commanders. By at least 300 BC the title of Consul was being used. Consuls had extensive powers in peacetime (administrative, legislative and judicial), and in wartime often held the highest military command. Consuls also read auguries, an essential step before leading armies into the field.
To prevent abuse of power, the office was shared by two consuls, each of whom could veto the other’s actions. Consuls were elected by the Comitia Centuriata. However, they formally assumed powers only after the ratification of their election in the older Comitia Curiata, which granted the consuls their imperium. The consuls were invested with the executive power of the state and headed the government of the Republic. Their position as chief judges was transferred to the praetors in 366 BC. After this time, the consul would only serve as judges in extraordinary criminal cases when called upon by the Senate.
The consuls acted as the head of government, and all the other magistrates, with the exception of the tribunes. The internal machinery of the Republic was under the consuls’ superintendence. As part of their executive functions, the consuls were responsible for carrying into effect the decrees of the Senate and the laws of the assemblies. Each consul served as president of the Senate for a month. They could also summon any of the three Roman assemblies (Curiate, Centuriate, and Tribal) and presided over them. When neither consul was within the city, their civic duties were assumed by the praetor. Outside the walls of Rome, the powers of the consuls were far more extensive in their role as commanders-in-chief of all the Roman legions. When legions were ordered by a decree of the Senate, the consuls conducted the levy in the Campus Martius and led the forces in the field.Within the Modern Res publica:
The consuls act the joint heads of the republican government. They can summon the senate and introduce new legislation. They can also call the Curiate, Centuriate, and Tribal assemblies to order. Like in antiquity the consuls oversee the daily operation of the government and all magistrates. They have the power to veto all regularly elected magistrates with the exception of the tribunes. They also have the power to oversee extraordinary judicial cases brought to the senate for appeal. Lastly, the consuls act as the main representatives of the res publica government and obersee the daily operation of the Res Publica.
In the res publica two Consuls are elected for a term of one year by the Centuriate Assembly.