The Senate of this period was made up of three factions, each amounting to one third of the voting power: conservatives who supported the Republic as it had always been, moderates including Cicero and Cato who allowed some adaptation of the political system, and the liberals who supported Pompey and Caesar. The conservatives were so strict in their point of view, they tried to block all efforts of the triumvirs, unable to perceive the harm they would eventually bring to themselves.
Pompey and Crassus decided to bury the hatchet and go in with Caesar. The latter was still the least influential of the triumvirs but he had two important assets: he was by far the best negotiator and he had previously been a supporter of Marius, the man of the people, whereas the other two were seen as allies of dictator Sulla.