Plebeian tribunate
In the year 51 BC, he prepared to stand for the aedileship the following year. But after the conviction of one of the plebeian tribunes-elect in July, he took the opportunity to stand as that tribune's replacement. The same year, he was elected to the pontificate. His political position had been firmly anti-Caesarian and he was expected to support Caesar's removal from Gaul without honours, block a possible second consulship, and repeal Caesar's agrarian legislation in 59. However, Curio changed his views, possibly because he resented the senate's refusal to insert an intercalary month, or after receiving a massive bribe from Caesar. However, it did not become clear that he had become an ally of Caesar for some months; Curio continued his anti-Caesarian proposals until his tribunician proposals – including for annexation of Mauritania – were rejected by the senate.
Only in March, when the question of Caesar's command was mooted before the senate, did Curio's position become more clear when he demanded that if Caesar were to be removed in Gaul, Pompey must also be removed in Spain. Through the year, Curio vetoed any other discussion of Caesar's command. The proposal was widely praised by the general population as an acceptable compromise that would avoid civil war, giving Curio tremendous popularity, but at the same time winning Curio the enmity of Pompey. The proposal also won the approval of many senators, who viewed it as "more attractive... than the rhetoric of inflexible confrontation".
In December 50, one of the then-censors, Appius Claudius Pulcher, attempted to have him removed from the senate, a proposal which the senate rejected at Curio's urging. At a following debate on Curio's motion that both Caesar and Pompey should step down, the senate voted hugely in favour (370 to 22). This motion, however, was vetoed by the consul, who then extrajudicially called upon Pompey to raise men to fight Caesar. Upon the expiration of his tribunate on 9 December 50 (tribunes took office on 10 December rather than 1 January), he complained to the people about Pompey and consul Marcellus and promptly fled to Caesar in Ravenna.
As the year drew to a close, relations between Caesar and Pompey drew to a breaking point: a last-minute proposal brought by Curio and two of the tribunes for that year (Mark Antony and Quintus Cassius Longinus) on 1 January 49 BC was rejected and the senate – at the urging of the hardliners – voted to remove Caesar from his command (Curio and Marcus Caelius Rufus were the only dissenting votes) and moved the senatus consultum ultimum against Caesar. In response, Curio, Antony, Cassius, and Caelius fled the city to Caesar, and he then took up arms against the senate.
