de summa re publica, 'on a matter so vital to the state.' Cf. [1. 14] 'ad summam rempublicam pertinent.'
principibus, the leaders.
sententiae. See [on 1. 9].
senatus consultum, the regular word for a decree of the senate. When passed, it was written down, and the names of the principal supporters appended.
liberata . . . sit, etc. Subj. because he is quoting the substance of the decree. usus essem is plup. because 'laudantur' (historic present) is regarded as a past tense.
viro forti, collegae meo. This was C. Antonius Hybrida. He had been associated with Catilina, but Cicero induced him to come over to the side of the senate by giving up to him the province of Macedonia. See [note on 4. 23].
a suis et rei publicae, etc., i.e. he had ceased to hold any communication with them, either on his private affairs, or his public duties.
cum se praetura, etc. Note the construction of 'abdico.' Strictly speaking, a magistrate could not be deprived of his office except by his own act; but resignation was practically compulsory under certain circumstances (e.g. when the election was proved to have been obtained by bribery). As praetor, Lentulus could not have been placed under arrest, for the person of a magistrate was inviolable (sacrosanctus).
in custodiam. See [on 1. 19].