Three differences between the roman republic government and the United states

Universal Suffrage: Rome only gave the citizenship and the voting power to the upper wealthy class but in USA every citizen has the voting power regardless of the social class.

Electoral college system and protection for minorities: Rome was a Republic and was governed only by the majority rules. However, in US governmentsystem, specific provisions Andre placed to protect the interests of the minorities as well.

Roman Republic was a centralised government with all the decision making power was with the central government. However, US system is a Federal government system in which the state governments have a considerable decision making power as well.