During the course of the 3rd to 2nd centuries BC, the Roman Republic rapidly expanded its territory across the entire Italian peninsula and into far off lands such as Spain and Greece. Particularly in Italy, the conquests involved the seizure of a lot of farmland that could not be implemented into the Roman colonial system in a timely manner, so a lot of it simply became public land for anyone to farm on if they lacked their own land. This public land was known as the ager publicus.
By the mid 2nd century, there began an internal crisis within the demographics of Rome. Military campaigns dragged on in Spain, which was a lethal area that did not contain much wealth to be looted. Soldiers could be called to serve multiple years to keep force cohesion in the faraway Spanish campaigns. This caused a huge problem. During this time, Roman soldiers had to purchase their own equipment, and there was a property wealth requirement to be able to serve in the army. Since young men in their prime were being sent to multi-year long campaigns in poor provinces, Roman farmers began to steadily lose wealth, as the most productive laborer (the young man) was away for many years, causing the household to lose income and possibly take out loans. When the soldier returned to his farm, he would have been much poorer than when he left, especially since there was no booty in Spain. Many could not repay their debts and had to sell their land.
In theory, the ager publicus system was supposed to be a solution to this problem. Any farmer who fell on bad times could work on the public lands until they had saved enough to buy their own land. However, this was not actually possible. The wealthy senatorial and equestrian classes squatted on the public lands, especially when they were adjacent to their own properties, and created huge super-estates worked by slaves, primarily for the production of olives or grapes. The majority of the land began to be monopolized by the elites. Since the ager publicus was not available, the destitute had to move to urban areas, such as the city of Rome, where their limited skill set as only a farmer and a soldier gave them poor employment prospects. These people would only be able to find work as seasonal laborers and could barely keep themselves alive.
Tiberius Sempronius Gracchus foresaw two issues (other than a bunch of people starving) with this. One, that Rome would soon face a demographic crisis as the majority of the urban poor could not afford to have numerous children, and two, that the lack of propertied men would result in too few men for the army.
Gracchus becomes elected as tribune of the plebs for 133 BC. He proposes a land reform bill to the Senate. This bill would start enforcing a 120 hectare land limit and implement a commission to distribute land to the urban poor. The senators, being the principle offenders of the monopolization of land, immediately shoot it down, so Gracchus goes directly to the Plebeian Assembly.
Technically, legislative power was given to the Plebeian Assembly. The Senate was supposed to only be an advisory body. For the entire early Republic, this seems to have actually been the case. However, during the 2nd Punic War, the emergency allowed/required the Senate to take on more prerogatives in regard to legislation, and the assemblies were regulated to mere rubber stamps on the Senate's laws. Gracchus was technically not doing anything illegal by going to the assemblies and he was acting in accordance to earlier tradition.
Although the Senate tries to bribe another tribune to veto the bill in the assembly, Gracchus has the assembly remove the tribune from office and passes the agrarian bill. A new commission made up of Gracchus, his brother Gaius, and his stepfather Appius Claudius Pulcher is formed to distribute some land. It seems to have been less effective than was hoped.
With the passing of this law and especially the side-stepping of the Senate, the elites were absolutely furious with Gracchus. Gracchus was not the first person to propose land reform. A few years earlier, Gaius Laelius proposed a similar bill, but the Senate made it known that they would kill him if he didn't drop it. He did drop the bill, which earned him the title from his peers Sapiens - the wise. With this in mind, Gracchus had to protect himself from both lawsuits and assassinations after his term as tribune ended.
A tribune is immune to lawsuits during his time in office and his body was consider inviolate; it would be an offense to the gods if someone hurt a tribune. Gracchus needed to stay as a tribune for as long as possible to protect himself. He decided to run for reelection, which was highly against tradition, giving the Senate grounds to take action. The Senate bribed people to obstruct the reelection of Gracchus, but Gracchus, who by now had bodyguards, were able to overpower them and throw them off the podium, allowing himself to be reelected.
Meanwhile in the Senate, the senators are losing their minds. They try to convince the consul to arrest Gracchus, but he refuses to take action. Then, the Pontifex Maximus, or the chief priest of Rome, says, "Anyone who wants to save the community, follow me." Some senators support him, and they begin to break up the furniture in the senate house to acquire wooden cudgels. The Pontifex Maximus pulls his toga over his head, which is usually only done when making sacrifices to the gods. He leads a senatorial mob to Gracchus and slaughters him and his supporters. This was the first political assassination in Rome and marked the beginning of a decline.
Although Gracchus was dead, others would continue his legacy. His brother Gaius Gracchus would continue some of his same policies until his own death, and Marcus Livius Drusus would also continue them until he was also killed. The infamous Gaius Marius would pretend to be a populist politician in the style of Gracchus, paving the way for his nephew - Julius Caesar - to do the same.