Division of Government Branches
The other important thing the Romans brought to today’s democracy is the division into executive, legislative and judiciary government branches. Even though most of the elected officials were selected from the pool of the members of the Senate, which was the legislative authority, during their terms on other functions, officials were unable to affect the work of the Senate.
Similar rules are usually present in every modern country, where various legal mechanisms prevent politicians from taking multiple position within the system.
In fact, Romans believed that by separating the positions so strongly, not allowing them to be taken by the same person, would protect the Republic from devouring itself. The transition into the Empire began with Cornelius Sula, and then continued with Caesar, who used their military strength to amass civilian power.