The extent of the power of the Senate over legislation varied greatly in different periods of Roman history, and these differences were caused more by the existing political conditions, and by the relative strength of the aristocratic and popular parties in Rome, than by any express changes by legislation.
At the very outset of Roman history we see the Senate existing as an aristocratic body, embodying in itself both the oligarchical principles upon which the Roman government was based, and also the patriarchal basis upon which the Roman family organization and later the organization of the Roman state itself had been built.
Originally, each of the three Roman tribes was divided into ten gentes, each gens into ten curiæ, and each curia, besides constituting one of the units in the comitia curiata, furnished one member of the Roman Senate. The Senate continued after the organization by curiæ had become obsolete. Membership in the Senate was at all periods for life, but did not descend from father to son. Vacancies in the Senate were filled by appointment, these appointments being made first by the kings, later by the consuls, and finally by the censors. As the censors were chosen only once in five years, vacancies in the Senate were filled only at such intervals. The aristocratic party in Rome, by keeping control of the office of censor, was able to perpetuate their majority in the Senate. In filling such vacancies, preference was given to those who had held some of the higher offices during the preceding five-year-period. Many members of the Senate had held the office of consul; many more hoped to hold it in the future. All members of the Senate, with few exceptions, had held some civic office, and were men of property and of mature age.
All the dignity of Rome and of the Roman government centered in the Roman Senate. The minister of Pyrrhus described this body as "an assembly of kings," and it might well have aroused the surprise and admiration of a foreign ambassador, as nowhere else in the world at that time was it possible to find such an assembly, either from the standpoint of the character of the body itself or of the qualifications of its members.
At an early period no law could be presented before the comitia centuriata or the comitia tributa without having been previously approved by the Senate, and after the passage of the act, either by the comitia centuriata or the comitia tributa, it must be promulgated by the Senate before it went into effect. The Senate, therefore, was never possessed of a direct general power of legislation, but had in the fullest degree both the power of initiating legislation and of vetoing it. At a later period the control of the Senate over legislation became theoretically less, but practically greater.
By the Publilian Law (339 B.C.) the control of the Senate over the comitia centuriata was reduced to a mere formality. By this time, however, the officers of the state, the tribunes as well as the consuls, had fallen completely under the control of the Senate, while the comitia tributa, in turn, fell more and more under the control of the consuls and tribunes respectively. During the latter period of the republic the Senate practically legislated, and gave the bill to one of the tribunes (the tribunes were at this time far more completely under the control of the Senate than were the consuls) to secure the mere formality of its passage by the comitia tributa.
The management of foreign affairs was at all times exclusively in the hands of the Senate, except that the question of declaring war or concluding peace must be submitted to the vote of the people in one of the popular assemblies. The Senate also regulated the religious affairs of the Roman state (after this power fell from the hands of the comitia curiata); assigned consuls and prætors their provinces of administration and command; fixed the amount of troops to be raised both from the Roman citizens and from the Italian allies; sent and received ambassadors; controlled the calendar, adding to or taking away from a year so as to lengthen the term of a favorite official or to shorten the term of an unpopular one; decreed or refused triumphs to Roman generals, and possessed a general control over the financial affairs of the state.