Oh. Rubicon. There are better treatments of the subject, in fact I'd go so far as to say that I don't know of any worse treatments.

Cato could hardly take offense against the crossing of the Rhine in 58, since it hadn't happened yet. Caesar had violated several laws during his consulship. Importantly, he had violated Bibulus' right to obnuntiatio and subjected him to physical violence. Though one of the leges Clodiae in 58 abrogated the law de obnuntiatione there is no evidence that it was retrospective. Caesar's establishment of the decemvirate and the Eastern Settlement were not altogether legal actions, being undertaken in the face of important opposition and (by detaining Cato with no good cause) possibly without legality, and though it was a somewhat thornier point than his violation of Bibulus' consular privilege it was still grounds for prosecution. The senate ordinarily held the right to assign proconsular provinces, but Caesar's ally Vatinius--supported by Pompey, thanks to the very-much-illegal triumvirate--successfully forced through a tribunician bill of questionable legality granting Caesar Cisalpine Gauley and Illyricum, to which Transalpine Gaul was added later. With these came the immunity of extended provincial imperium, the same guardian that Caesar secured again at Luca, despite the senate's initial resolution to grant him a proconsular command over the pastures and forests of Italy, a traditional office that came without imperium--Caesar's rapid flight into his province should be more than enough proof of the lex Vatinia's potency. It was a questionable bill, and Vatinius was prosecuted de ambitu at the first opportunity, during his bid for the praetorship in 55. Notably, Publius Clodius early in 58 threatened to abrogate all of Caesar's legislation--the legality of Caesar's consulship, and the legislation passed during it, was very much open to question, and he had certainly broken the law in his action against Bibulus.

The consular elections for 59 were particularly sordid, but that does not mean that all elections were marred by prominent wrongdoing. Roman law established prohibitions against ambitus, but elections were expensive and a certain amount of money was expected to be parceled out by a candidate. The nature of the nobilis' place in Roman society was one of patronage, supplying clients and clients-to-be with social status and, along with everything else, money. The candidate had an obligation to display prominently his magnificence to the community--he need not greatly exceed his regular duties as a patron, except perhaps in that he was applying to a much greater number of people, and excessive campaigning, and the bribery that came with it, was cause for prosecution. There was little in the elections for 59 that was usual

I'm not sure why Caesar's dictatorship is being brought up in relation to his consulship a excuse earlier. Have you read Plutarch or Suetonius? The comparison to Sulla is central to their depiction of Caesar