A constitution is the basic law of any land, and when and if it's breached or treated with contempt, the center cannot hold. The result is turmoil, civil strife and, usually, the institution of one-man or one-party rule.
Republican Rome is a case in point. The bloody contests for supreme power in the state were never actually between the people and the senate but between two sets of wealthy aristocrats with the common citizens furnishing the spear-fodder for each side. Some grandees assumed the title of populares, champions of the poorer people and disaffected businessmen demanding social change. Others, conservatives, called themselves optimates, or the "best men" who defended the scheme of things against any radical reform.
After a century of relative amity during which Rome conquered its world, the Gracchi, brothers born at the core of Roman nobility, tried to initiate reforms to aid the swelling proletariat by awarding public lands (then leased to rich men) to landless peasants. The rationale was not socialism but to strengthen Rome's military manpower, because only property-owning citizens were constitutionally permitted to bear arms. Reform was certainly needed for both military and social reasons.
However, the Roman constitution stood in the way. Having driven out their kings, Romans established a republic with collegial officers elected for a single year — to prevent abuse of power and one-man rule.
When T. Gracchus was elected tribune, the most effective legislative office, the senatorial party was able to elect a conservative colleague. This meant that neither tribune could do anything in opposition to the other; it prevented one-man rule but paralyzed the state. Further, since magistrates held office for only one year, there was little chance of enacting sweeping new legislation.
Historians generally agree that Gracchus, frustrated, seized upon unconstitutional methods. Using emotional, rabble-rousing rhetoric, he led the people to oust M. Octavius, his colleague, and rammed his new laws through over the objections of the senate. Then, against all republican tradition, he stood again for office, to prevent likely repeal.
This infuriated the conservatives, and Tiberius Gracchus, like his brother earlier, was killed in rioting. But the damage was done: The constitution had been breached, an example for ambitious and unscrupulous men.
The Gracchi are hard to evaluate. They won posterity's sympathy by their idealistic goals, but their methods were atrocious. Tiberius was prideful, demanding the dues of his noble station, imperious, but also willing to arouse the mob.
Gaius and Tiberius Gracchus misunderstood the real situation in Rome. The government itself had to be reformed before real change could be carried out. Aside from simply passing controversial laws, they had no clear purpose as to that except for vague notions of installing Greek-style democracy, which had already failed in the Hellenic world. What they did was arouse vast passions and provoke the first political murders of Romans by Romans and eventual civil war.
Soon after, Marius, another popular leader, enlisted the propertyless proletariat in his army, the state now furnishing arms. In a time of emergency, his reforms, although against the established law, stuck. But again there was civil war, and now thousands of citizens were killed.
During this last century of the Republic, hatred and bloodshed between the partisans, the optimates and populares, became institutionalized. Neither side seemed to care much why they fought; families changed sides because of relationships and changed back for convenience. Related to Marius, the princely house of the Julii were Populists; the newly ennobled Cicero a staunch defender of senatorial power. Elections became thoroughly corrupt (politicians bankrupted themselves to gain office), and few leading figures respected either the constitution or the law.
The end, of course, was dictatorship and finally imperial monarchy, by which all classes lost their former liberties.
Historically, there's always a danger of this happening when partisans dig in, automatically opposing each other whatever the issue. The more even the balance of power, the more bitter the jockeying to grab it. Majorities and minorities can work out deals, but when two political sides are equally, frustratingly powerful and develop generational hatreds, something is bound to break.
It's nice when they get it right ...