The first oration of Calidius of which we have mention was delivered in B. C. 64, when he accused Q. Gallius, a candidate for the praetorship, of bribery. Gallius was defended by Cicero, of whose oration a few fragments are extant. (Ascon. in Orat. in Tog. cand. p. 88, ed. Orelli; Cic. Brut. 80 ; Festus, s. v. Sufes.) In B. C. 57 Calidius was praetor, and in that year spoke in favour of restoring the house of Cicero, having previously supported his recall from banishment. (Quintil. x. 1.23 ; Cic. post. Red. in Sen. 9.) In B. C. 54, he defended, in conjunction with Cicero and others, M. Aemilius Scaurus, who was accused of extortion. (Ascon. in Scaur. p. 20.) He also spoke in the same year on behalf of the freedom of the inhabitants of Tenedos, and in support of Gabinius. (Cic. ad Q. Fr. 2.11, 3.2.) In B. C. 52, Calidius was one of the supporters of Milo, after the death of Clodius (Ascon. in Milon. p. 35); and in the following year (51) he was a candidate for the consulship, but lost his election, and was accused of bribery by the two Gallii, one of whom he had himself accused in B. C. 64. (Cael. ap. Cic. ad Fam. 8.4, 9.)

In the debate in the senate at the beginning of January, B. C. 49, Calidius gave it as his opinion that Pompey ought to depart to his provinces to prevent any occasion for war; and on the breaking out of the civil war immediately afterwards, he joined Caesar, by whom he was appointed to the government of the province of Gallia Togata. He died at Placentia, in his province, in B. C. 48. (Caes. Civ. 1.2; Euseb. Chron. Ol. 180. 4.)