generibus, ordinibus, to all parties, classes.
non modo quam putaram: 'to say nothing of what I had anticipated.'
populares: the triumvirs, who found their supporters in the democratic party.
Bibulus: Caesar's colleague in the consulship; cf. Lucceium, Ep. VI.11n. He opposed Caesar's plans to the best of his ability, but his opposition was rather obstinate than effective. Cf. Mommsen, Rm. Hist. IV. 245. Subsequently he was commander of Pompey's fleet in the Civil War.
in caelo est: is extolled to the skies.
unus homo, etc the celebrated line from the Annals of Ennius descriptive of Fabius Maximus Cunctator, ironically applied to the passive resistance of Bibulus.
ipse se adfixit: i.e., by allying himself with Caesar.
tenent: sc. the triumvirs.
illa causa: the cause of the triumvirs.
illam amicitiam: Cicero's well-known friendship for Pompey.
utor via: i.e. the via media, turning off neither to the one side nor to the other.