The populares (/, /; Latin: populares, "favouring the people", singular popularis) were a grouping in the late Roman Republic which favoured the cause of the plebeians (the commoners), particularly the urban poor. They supported laws regarding the provision of a grain dole for the poor by the state at a subsidized price. They wanted reforms which helped the poor, particularly redistributing land for the poor to farm, and debt relief. At times they also supported the extension of Roman citizenship to Rome's Italic allies. A popularis was a politician who supported this faction. The populares are regarded in modern scholarship as in opposition to the optimates, who are identified with the conservative interests of the patricians (the aristocracy) and supported the senate, which represented its interests. The collapse of the higher class's ability to manage and govern Rome helped the populares become a party. New competition and campaigning started as a result to help support the masses and thus the populares were born.
The plebeian tribunes (the representatives of the plebeians) and the Plebeian Council (the assembly of the plebeians) at times clashed with the senate over the mentioned reforms and over the power relationship between the plebeian institutions and the senate. The optimates among the senators spearheaded the senatorial opposition. These tribunes were supported by populares politicians, such as Gaius Marius and Julius Caesar, who were often patricians, or equites (equestrians, the second social rank in Rome).
Conflicts between populares and optimates also played a part in some of the civil wars of the late Roman Republic: Sulla's First Civil War (88–87 BC), Sulla's Second Civil War (82–81 BC), the Sertorian War (83–72 BC), Lepidus' Rebellion (77 BC), Caesar's Civil War (49–45 BC), the Post-Caesarian Civil War (44–43 BC), the Liberators' Civil War (44–42 BC), and the Sicilian Revolt (44–36 BC).
The populares reached the height of their ascendancy four times. The first one was with the Gracchi brothers, who mobilised the plebeians in support of their land reform and their challenge to senatorial supremacy (133 BC and 122 BC). This almost was not the issue, because the populares had help from the Italians and they had to offer more land to the Italians than they wanted to. The land was meant to go to the poor. The second time was with Gaius Marius and his son Gaius Marius the Younger, when the Marians (the supporters of Marius, who were populares) seized power and held Rome from 87 BC to 82 BC. They were defeated in Sulla's Second Civil War. This was caused by the deteriorating relationship between Sulla and Marius, they started to fall apart from the consul stand point and lost view of what was important. The third time was when Julius Caesar was elected as consul in 59 BC with the support of Marcus Licinius Crassus and Gnaeus Pompey, who formed an informal alliance with Caesar which historians call the First Triumvirate (60–53 BC). These three helped bring Rome back to order from the scene it was when Sulla had left. The First Triumvirate gave each of the men their own land to watch over so parts of Rome were equally distributed among themselves so none of them would have too much to rule over. Caesar managed to pass an agrarian law for a land reform. This had not been achieved since the agrarian law of Tiberius Sempronius Gracchus of 133 BC. All subsequent attempts at agrarian legislation had been thwarted by the opposition and obstructionism of the optimates. Tensions between populares and optimates had increased with the Catiline conspiracy (63 BC) against the consulship of Marcus Tullius Cicero (an optimate) during which Cicero, supported by a final decree (senatus consultum ultimum) of the senate, had some of the conspirators executed without trial. There were demonstrations against these summary executions and this display of arbitrary senatorial power. There were two attempts to counter senatorial dominance which failed, but were popular. The proponents were Quintus Caecilius Metellus Nepos Iunior, a plebeian tribune, and Caesar, who at the time was a praetor. This enhanced Caesar's popularity and was a help for his creation of the First Triumvirate three years later. The fourth time was with Caesar's Civil War, when Caesar held power from 49 BC to when he was assassinated in 44 BC. Caesar relied on the support of the people for his power. After the defeat of Sextus Pompey (the son of Pompey) in the Sicilian Revolt by the Second Triumvirate in 36 BC, popularis ceased to be a relevant political label.
Notable populares included men who held the plebeian tribunate such as the Gracchi brothers, Gaius Papirius Carbo, Lucius Appuleius Saturninus, Marcus Livius Drusus, Publius Sulpicius Rufus, Servilius Rullus and Publius Clodius Pulcher; and men who held the consulship, such as Appius Claudius Pulcher, Publius Mucius Scaevola, Marcus Fulvius Flaccus (who also became a plebeian tribune), Gaius Marius, Gaius Marius the Younger, Lucius Cornelius Cinna, Gnaeus Papirius Carbo, Marcus Aemilius Lepidus and Julius Caesar. There were other notable populares, such as Quintus Sertorius who participated in the capture of Rome by the Marians in 87 BC and fought the Sertorian War, Marcus Aemilius Lepidus and Marc Antony, who fought for Caesar, were given a consulship by him and later became members of the Second Triumvirate. Although Marcus Licinius Crassus did not play a prominent part in Roman politics apart from his consulship in 70 BC, prior to being part of the First Triumvirate he was known as a supporter of the populares. Gnaueus Pompey was also a member of the First Triumvirate. The optimates in the senate side-lined him and frustrated his attempts to have his settlements in the east after his victory in the Third Mithridatic War ratified and to promote an agrarian reform to redistribute land to his veterans. Pompey's attacks pushed back Mithridates and Pompey even managed to get Mithridates's son to become an ally of Rome. As a result, he joined forces with Caesar and Crassus. After the death of Crassus, Pompey drifted towards the optimates. These shifting allegiances are reminders that the designation populares refers as much to political tactics as to any perceived policy. Holland notes that Republican politicians 'had always been more divided on issues of style than of policy.' 
A historian of the Late Republic cautions against understanding the terms populares and optimates as formally organized factions with an ideological basis:
|“||Our chief contemporary witnesses to the political life of the late Republic, Cicero and Sallust, are fond of analyzing the political struggles of the period in terms of a distinction between optimates and populares, often appearing with slight variations in terminology, such as Senate, nobility, or boni versus People or plebs. But what precisely is denoted and connoted by this polarity? Clear enough, one who is designated in these sources as popularis was at least at that moment acting as 'the People's man,' that is a politician — for all practical purposes, a senator — advocating the rights and privileges of the People, implicitly in contrast to the leadership of the Senate; an 'optimate' (optimas), by contrast, was one upholding the special custodial and leadership role of the Senate, implicitly against the efforts of some popularis or other. The polarity obviously corresponds with the dual sources of institutional power in the Republic — Senate and People — and was realized in practice through contrasting political methods … and distinctive types of rhetorico-ideological appeals suited to tapping those alternative sources of power … . It is important to realize that references to populares in the plural do not imply a co-ordinated 'party' with a distinctive ideological character, a kind of political grouping for which there is no evidence in Rome, but simply allude to a recognizable, if statistically quite rare, type of senator whose activities are scattered sporadically across late-Republic history … The 'life-long' popularis … was a new and worrying phenomenon at the time of Julius Caesar's consulship of 59: an underlying reason why the man inspired such profound fears.||”|
This summarizes the dominant interpretation of the populares in 20th-century scholarship, deriving in large part from Ronald Syme in the Anglophone literature. In the early 21st century, and as early as the publication of the ninth volume of The Cambridge Ancient History in 1994, the validity of examining popularist ideology in the context of Roman political philosophy has been reasserted. T.P. Wiseman, in particular, has rehabilitated the use of the word "party" to describe the political opposition between optimates and popularists, based on Latin usage (partes) and pointing to the consistency of a sort of party platform based on the food supply and general welfare of the populus, making land available to those outside the senatorial elite, and debt relief.