The Constitution of the Roman Republic was a set of guidelines and principles passed down mainly through precedent. The constitution was largely unwritten and uncodified, and evolved over time. Rather than creating a government that was primarily a democracy (as was ancient Athens), an aristocracy (as was ancient Sparta), or a monarchy (as was Rome before and, in many respects, after the Republic), the Roman constitution mixed these three elements, thus creating three separate branches of government. The democratic element took the form of the legislative assemblies, the aristocratic element took the form of the Senate, and the monarchical element took the form of the many term-limited consuls.
The ultimate source of sovereignty in this ancient republic, as in modern republics, was the demos (people). The people of Rome gathered into legislative assemblies to pass laws and to elect executive magistrates. Election to a magisterial office resulted in automatic membership in the Senate (for life, unless impeached). The Senate managed the day-to-day affairs in Rome, while senators presided over the courts. Executive magistrates enforced the law, and presided over the Senate and the legislative assemblies. A complex set of checks and balances developed between these three branches, so as to minimize the risk of tyranny and corruption, and to maximize the likelihood of good government. However, the separation of powers between these three branches of government was not absolute; and moreover, several constitutional devices that were out of harmony with the Roman constitution were used frequently. A constitutional crisis began in 133 BC, as a result of the struggles between the aristocracy and the common people. This crisis ultimately led to the collapse of the Roman Republic and its eventual subversion into a much more autocratic form of government, the Roman Empire.