Sandefur conveniently ignores history by blaming the decline of the Republic on the populares. The decline of the Republic is properly laid on the shoulders of the conservative rump of the Senate, those who took over after Sulla, including Lucullus, Pompey, and Crassus. Despite popular mythmaking, Caesar was politically unimportant until his consulship, which came only after the Catilinarian conspiracy.

It is true that Caesar opposed the death penalty for the Catilinarian conspirators (along with many other Senators), but the only evidence for Caesar's complicity in any acts of Catiline come from Cicero and Sallust, who were hardly pro-Caesar, and who had much to gain from exalting the conspiracy beyond its actual importance. Once the attempt to assassinate the consuls was discovered, Catiline was gone for good.

Until Caesar crossed the Rubicon, he was no different than any other highly successful Roman general and governor, and certainly a great deal less offensive to Rome than Pompey or Crassus, both of whom illegally marched on Rome and were saved from prosecution and exile only because they both became consul the next year and legislated themselves immunity through the Popular Assembly. Crassus and Pompey were the two wealthiest Romans of the era (excluding Brutus, who inherited the Gold of Tolosa from his half-uncle, Quintus Servilius Caepio); Caesar was destitute when he left Rome to take up the governorship of Gaul.

Once Caesar crossed the Rubicon, all bets were off, and Sandefur is right: Caesar was not good. But Cato had hated him for years previous to his clearly unconstitutional crossing of the Rubicon, as had Bibulus. I don't see how Caesar suddenly acting unconstitutionally for the first time in his career ratifies Cato's opposition to him. Cato was far less of a defender of freedom than he was a man who opposed Caesar. Cato's initial claim to fame was that he prevented the removal of a structurally unnecessary pillar from the Basilica Porcia, where the ten tribunes of the plebs met, for no other reason than that his grandfather, the Censor, had built it that way, and thus it had become part of Rome's unwritten consitution. Let's not forget that Cato tried to kill Metellus Scipio and Aemilia Lepida after Aemilia Lepida refused to marry Cato, and that Cato sued Scipio for alienation of her affections, even though Aemilia Lepida had been engaged to marry Metellus Scipio for years.

Cato was a crackpot loon who hated Caesar. Caesar was a megalomaniac who had done nothing unconstitutional until he crossed the Rubicon. Cato does not get a free pass just because he opposed Caesar.

Now, Cicero is the man who should be highest in the firmament of those who defended the Republic.

EDIT: as for why Caesar's march was justified versus the Senate's decree, Sandefur forgets that 1) the people, not the Senate, were sovereign, and the Senate ignored Mark Antony's tribunician veto of the decree stripping Caesar of command; 2) the Plebeian Assembly later ratified Caesar's acts. The most recent decision governs, as we know.