In the marketplace, a collection of citizens discusses Coriolanus's candidacy, saying that if he uses the scars of battle in his appeal to them, they will probably make him consul. Then, Coriolanus himself comes in, accompanied by Menenius, who offers encouragement and then leaves his friend alone with the crowds, which come to him in small groups. Coriolanus struggles and cannot conceal his customary arrogance, but by calling attention to his military service, he manages to convince a large body of the citizens to vote for him. Brutus and Sicinius reluctantly acknowledge that he has passed the test, and Menenius leads him back to the Capitol to be invested with the robes of office.
When Coriolanus has gone, the plebeians remark on his arrogance, and the two tribunes demand to know why they voted for such an arrogant patrician. The plebeians decide to retract their approval and deny Coriolanus the consulship; elated, Brutus and Sicinius tell the crowds to gather their friends and go to the Capitol. Covering their own backs, the two tribunes advise the crowds to say that they only voted for Coriolanus because the tribunes told them to and that now they have come to their senses and want to have the vote rendered invalid.
Meanwhile, on the Capitol, Titus Lartius tells Coriolanus that Tullus Aufidius has raised a new army. Coriolanus worries that the Volsces will attack Rome despite the newly signed peace treaty, but Lartius assures him that they have been broken and will not fight again. At that moment, the two tribunes arrive and tell the assembled Senators that the people of Rome will not accept Coriolanus as consul. Furious, Coriolanus accuses Brutus and Sicinius of rallying the plebeians against him and then begins to denigrate the common people, warning his fellow patricians that allowing the rabble to hold power, to have tribunes, will ultimately lead to the downfall of the Senate. Menenius urges him to return to the market and beg the people's pardon, but Coriolanus refuses and continues to denounce the plebeians--and the patricians, for having ever agreed to allow them a share in Rome's governance. Brutus and Sicinius accuse him of treason and call in a crowd of plebeians to seize him. He raves at them, and the two tribunes declare that he must be executed; in response, Coriolanus draws his sword, and the Senators come to his aid. Coriolanus and the Senators drive away the mob of plebeians, along with the two tribunes, and Coriolanus flees to a Senator's house. The mob returns in renewed strength, but Menenius convinces the people to allow him to reason with Coriolanus and to bring the great soldier to the market place for a public airing of all the grievances.
Later in the play, Menenius will remark that Coriolanus often thought of him as a father. The audience may scoff at the idea, since Volumnia is clearly father and mother both to her son, but the scene in the marketplace shows Menenius in a markedly paternal light, shepherding the reluctant Coriolanus down to meet with the people, only nervously leaving him alone to gain the necessary votes and then returning proudly when the unpleasant business is done. And this pride is not necessarily misplaced; for while Coriolanus does not do well--he is terribly uncomfortable in the role of supplicant, and his pride and contempt for the general public leak through his facade of humility-- he seemingly does well enough. Although the plebeians are determined not to make things easy for Coriolanus--they refuse to let him address them en masse--they do receive him with a certain generosity: They know what he has done for Rome, and they are willing to give him their votes despite his verbal fumblings.
Thus, Coriolanus has seemingly triumphed and hastily changes out of the garments and repairs to the Senate House, accompanied by a relieved Menenius. But the plebeians, despite their generosity, are not fools; they have noticed the thinly veiled contempt with which the candidate pleaded for their votes. It does not take much prodding from Brutus and Sicinius to make them change their minds, although the two tribunes work the crowd with political savvy; indeed, it is a stroke of political genius to have the crowds say that the tribunes pressured them into their original vote; Brutus and Sicinius appear as peacemakers.
The scene shifts to the Capitol, where the image of the assembled noblemen contrasts sharply with the crowds of plebeians from the previous scene; the contrast graphically illustrates the political division of Rome. The rumor that Tullus Aufidius has raised another army foreshadows the course that Coriolanus will take later in the play; but for now, it is a reminder of another kind of battle--the kind of battle the hero is better suited to fight. As long as he is on top, all goes well, but once Brutus and Sicinius arrive with the news that people wish to reverse their election, Coriolanus's behavior becomes disastrous. Instead of being conciliatory, he immediately goes on the attack, returning again to his old complaint about the absurdity of popular rule: "I say again, / in soothing them we nourish 'gainst our Senate / The cockle of rebellion, insolence, sedition / Which we ourselves have plowed for, sowed and scattered... (III.i.67-71)." From here, it takes only the slightest goading from the tribunes to drive him to declare that he means to "throw their power i' th' dust"(III.i.169); it is this treasonous outburst that moves them to threaten him with death.