I'm a Political Science student and during lectures about parliamentary history we were taught about so called factions that existed in Western European (semi-) democracies in the 19th century that existed before there were political parties. These factions were supposed so be groups of people that worked together in parliament around certain issues (economic, regional, etc.). I wondered whether such factions also existed during the Roman Republic.
This is a bone of contention because many Roman authors talk about "factions" or "parties" but modern scholars argue whether those words meant the same to them as they do to us. The biggest difference is that the parties mentioned by the Romans do not differ so much on policy as they do on method. The two "parties" were the populares and optimates and they really only differed on their respect for the Senate. Populares were comfortable ignoring the Senate. They would use plebiscites to enact legislation and were considered radical by the old Roman elite. They would whip up public support for sometimes radical legislation then use force to ensure their vote passed. The optimates, on the other hand, saw the Senate as the supreme power in Rome and used its authority to lead public votes.
Romans are hard to pin to "issues" in large part because their political system wasn't entirely devoted to it. For example, the arch-conservative Cato the Younger was perfectly comfortable using "polularis" techniques to prevent the Senate from doing anything to untraditional and supported the increase of the bread dole. On the other side, Cato balked at innovations like "extraordinary commands" voted on by the Senate or in plebiscites (these were commands like Pompey's command against pirates in 65 and giving Caesar control of Gaul through public vote in 59 and renewing it in 55). Likewise, Cicero argued vehemently against land reform, one of Pompey's biggest political goals, but also vociferously supported Pompey and Caesar's extraordinary commands. So even personal relationships did not totally align with political goals.
Readings:
For a discussion on the way Romans used "faction" and "party": Lily Ross Taylor, Party Politics in the Age of Caesar. Her reconstruction has been mostly torn to shreds by more recent scholarship (particularly PA Brunt) but this is such a fundamental work that it is used to frame the debate.
For the new consensus on Roman political and social obligations: PA Brunt, "Amicitia", "Gratia", and "Clientele" collected in The Fall of the Roman Republic and Related Essays.
The best overview of politics in Rome in the Late Republic is Erich Gruen's The Last Generation of the Roman Republic. He's something of a middle ground between Brunt and Taylor in the sense that he makes a political narrative borrowing from both of their ideas on Roman politics.
Most scholarship now is more concerned with individual politicians and how they gathered supporters or drummed up votes rather than "parties" or issue politics. A fairly recent example is Robert Morstein-Marx, Mass Oratory and Political Power in the Late Roman Republic. Another good one it Tatum, Patrician Tribune.