A Growing Divide
An important element of the story of Caesar and Pompey is the struggle between the populares1 and optimates2, championed by each of those men, respectively. A lot of historians leave this out, and refer to the factions as Caesarian11 and Pompeian2. The issue goes far deeper than that and I won't get into the full history here, perhaps in its own series... But suffice it to say that the divide was deep and deepening. The populares demanded drastic land reform to give power to the people working the farms and to soldiers. The optimates opposed radical progressivism and favored the 'old way' of things. That they called themselves the 'good men' I think speaks to their general attitude. They were keepers of something sacred, not politicians.
An Unlikely Alliance
From approximately 60-53 BCE Caesar and Pompey allied with Caesar's wealthy patron, Marcus Licinius Crassus. Crassus, like all Romans of standing, had served in the military, but was completely unremarkable. I think the most important tidbit is that he served with Cato.
Together, they collaborated to exert immense influence in the Senate, leveraging the power bases of both parties. This level of authority was not unprecedented, but it was certainly remarkable. This First Triumvirate3 dominated Roman politics for more than half a decade. Caesar was popular, Pompey had clout, and Crassus had money.
But these two great men did not start as friends and then split as Caesar campaigned in Gaul and Pompey in Asia; no, they were always at odds. Gaius Julius Caesar and Gnaeus Pompeius Magnus were never friends. That they formed two legs of the First Triumvirate was wholly an alliance of pragmatism.
It's important to note that while Crassus is often seen as a moderating influence between the two men, it seems to me that who he moderated was Caesar. Pompey was a little older and generally more restrained, his wars were legal and he aligned himself strongly with the Senate to reinforce mutual legitimacy. Caesar caused some, as we'll see, earth-changing wars for dubious reasons, and strongly aligned himself with the plebs, one of several patricians to do so.
The alliance was dubious to begin with and increasingly untenable as time went on. They created a status quo had to rebound from their concentrated influence once the alliance faltered. Pompey was a champion of the old guard while Caesar was a progressive populist. Together they dominated Roman politics for 7 years and created an entire national context, but it could never have lasted.
The Man in Black
Even as Caesar collaborated with a leading optimatis, he was constantly at odds with another: Marcus Porcius Cato Minor. Cato Minor, i.e. Cato the Younger, was something of an ascetic, a devout believer in the old ways of Rome. He disregarded comforts and when not in the Senate wore a black toga to symbolize his mourning for the Roman Republic.
Cato was known, during his command in Macedon, for leading his men from the front, sharing their work, food, and sleeping quarters. His discipline and punishment were harsh, but he was reportedly beloved by his legionaries. This would be lost in a later reputation as a diligent and effective administrator and vocal opponent of Caesar, but it tells about the character of the man.
In fact it would prove Cato who would be Caesar's final great adversary.
True Loyalty
There is an absurd amount to say about Marcus Tullius Cicero who was a statesman, lawyer, scholar and Academic Skeptic who was massively influential in the politics of the late Republic and upheld optimate principles during the crisis that led to the establishment of the Principate.
A note about his historic import: "His extensive writings include treatises on rhetoric, philosophy and politics, and he is considered one of Rome's greatest orators and prose stylists. His influence on the Latin language was immense: he wrote more than three-quarters of surviving Latin literature from the period of his adult life, and it has been said that subsequent prose was either a reaction against or a return to his style, not only in Latin but in European languages up to the 19th century."
Back on topic, Cicero, though known as an optimatis, was something of a centrist. He always championed republican government, but his loyalty always lay with the Republic, be it a true republic or a new monarchy, but he did so within his understanding of the most likely victor. He didn't play a large part in the beginning but as things developed his role as a devoted republican became ever more important.
He wasn't loyal to either leader. He wasn't even necessarily loyal to ideology. Cicero was loyal to Rome, the Roman Republic.
Remember his name.
Learn more about the background to Caesar's Civil War here. The impossible choice he had to make.
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Footnotes
1 - populares (literally 'favoring the people', singular popularis): theirs was the cause of the plebeian farmer; they saw the optimates as oppressive elitists
I think calling them the 'Caesarians' is fair, as he ran the party like a dictator throughout his Civil War
2 - optimates (literally 'best ones', aka boni, 'good men', singular optimatis): they favored the patrician class and the status quo, and saw the populares as dangerous radicals
Considering the continued influence of Cato in particular, I don't find it fair to call them either the 'Pompeians' or the 'Senate'. Pompey was not the sole authority, but Cicero believed he would become a military dictator, no different from Caesar.
3 - literally 'three men', in this case implying 'who are in power', as a near- rather than fully-monopolistic, bloc, this is not quite a true triarchy