Good request. Right off the bat, we have to ask what time period of Rome you are talking about. There were three stages of Roman development. First, they had kings, then they had a Republic, and finally they had an Empire. I will focus on the Empire, since it is what the Romans ultimately became.
Under the Empire, the Romans had an emperor, who had absolute power. What he said was done; it was law. He had little accountability and the senate did not really possess power at all. We can say that the Roman government was emasculated in view of the absolute power of the emperor.
Modern American government is quite different, because we do not have an emperor. The American form of government is based on a system of checks and balances. There is the Senate, the House, the Supreme court, and the President as well as the President's Cabinet.