Cicero's nemesis and a key political figure in the 60's BC was L. Sergius Catilina. Like Crassus, Catiline profited enormously from the proscriptions of Sulla's dictatorship, but Catiline was notorious for his brutality and unlawful methods. While a definite villain of the Late Republican period, he was also the victim of Cicero's eloquent attacks, which certainly did little to commend him either with his ancient peers or in modern review.
In 67 to 66 BC he served as the Propraetor governor of Africa and may have used his position to extort large sums of personal wealth. Upon his return to Rome, he sought the Consulship but accusations of provincial mismanagement and a general distrust barred his ability to run. In the year 65 BC, Catiline's efforts for the Consulship were blocked by impending trials for extortion, but massive bribery would eventually see his acquittal of the charges.
64 BC saw the election of Cicero as Consul. While Cicero, as a new man, wasn't overly popular with the Senatorial elite, he was a far better conservative alternative than the long line of corrupt individuals that sought the highest office. The choices for election in that year were Cicero, the despised Catiline, and his similar contemporary C. Antonius. To prevent a joint consulship with Catiline and Antonius, the Senate pushed Cicero forward along with Antonius to prevent the worst of three bad choices (in their opinion) from taking power.
During the Consulship of Cicero, he faced one early challenge of note. The agrarian bill of the tribune P. Servilius Rullus was intended to elect ten land commissioners with imperium. These commissioners would oversee all concerns with selling public land and distributions to veterans, with sweeping power. Cicero vehemently opposed the concept in light of the massive corruption of the time. Such positions would've undoubtedly been major sources of extortion and greed. The proposal was defeated in the plebeian assemblies, thanks in large part to Cicero's usual eloquent speeches. In a testament to his oratory skill and prestige, this was the first land reform bill that was ever defeated in a vote of the citizen assemblies.
For the year 62 BC, Catiline was again running for Consul and bribery was the operating standard of the day. Despite massive bribes, Catiline lost one again, however and certainly began to look at other measures to gain power. Shortly after the elections, a bundle of sealed enveloped were delivered anonymously to Crassus addressed to several Senators. In opening the one addressed to him, Crassus found that the letters were warnings, telling certain individuals that an impending massacre of many Senators was about to take place. Crassus took the letters to Cicero and it was discovered that they all said the same thing. Cicero then addressed the Senate with the plot and he was authorized with a tumultus, or a military emergency, to find the writer and uncover the plot. While Cicero tried to place the blame on Catiline, the author was never found, and it was suggested that the entire concept may have been hatched by Cicero himself as an excuse to prosecute troublemakers.