Cicero had several goals in his orations on Catiline. He was able to achieve these goals by the use of rhetoric. He manipulated the senate into believing something by playing on their preconceived fears of Catiline and his army.
Cicero claimed that Catiline was a madman, which destroyed his credibility, and made it easy for Cicero to denounce any counter-arguments he might have. Cicero accused Catiline of audacity, which prejudiced the senate against him. He hinted at Catiline’s murder of his wife, and his various personal vices. These accomplished the goal of fostering personal dislike of Catiline among the senators.
Catiline was already feared by the senators, so Cicero did not need convince them of how dangerous he was. All he needed to do was use this fear to control them. It was not difficult for Cicero to convince his audience that Catiline was the enemy of all Rome, and a danger to all of its citizens, because everyone was already afraid of him. Cicero used this fear to his advantage in achieving the goal of affirming the danger of Catiline, and positioning himself as the right person to liberate Rome.
Cicero used the senators’ fear of Catiline and his army to his advantage. If the senators were not already afraid of Catiline, Cicero would have had to start from scratch in convincing them of the danger surrounding him.