The early Republic was run by and for a narrow aristocracy. Winning it a broader base took two centuries of pressure from below, in a slow contest the Romans remembered as the Struggle of the Orders.
Patricians and plebeians
Roman society was divided between the patricians, a closed circle of noble families who monopolised the priesthoods and high offices, and the plebeians, everyone else: from poor labourers to wealthy landowners shut out of power by birth. The plebeians fought the wars and paid the debts but had little say in how the state was run.
The weapon of secession
Their great weapon was withdrawal. On several occasions the plebeians simply left the city in a body (a secessio) camping outside the walls and refusing to serve in the army until their grievances were met. A city that depended on their labour and their spears could not ignore a general strike of half its population.
The tribunes
The first and most important prize was the office of tribune of the plebs. Tribunes were sacrosanct (to harm one was a religious crime) and they wielded a startling power: the veto, intercessio, with which a single tribune could halt almost any act of the state to protect a citizen. It was a deliberate brake built into the machinery of government.
Written law
A second victory was the Twelve Tables around 450 BC, the first written Roman law code, set up in the Forum for all to see. Written law meant that judgments could no longer be invented on the spot by patrician magistrates; the rules were public and fixed.
A merged elite
Over time plebeians won access to the consulship and the other high offices, and the wealthiest plebeian families merged with the old patricians into a new governing class, the nobiles. The struggle did not abolish inequality (it broadened the elite rather than dissolving it) but it gave the Republic a wider foundation and a body of law that outlasted Rome itself.