Roman Senators had more power on their own than today's Senators who are often beholden to the wealthy benefactors who sponsor their campaigns. In Rome, powerful wealthy men similar to the Koch brothers would be much more directly involved. These incredibly wealthy senators would be able to use money or their influence to buy the support of the Praetorian Guard to help provide stability should their coup succeed. Today senators don't have that much power and they can't really compete with each other in the way that was happening in Rome. Also many politicians back then were Generals who's armies had more allegiance to them than the state itself (Caesar) who could seize power.