I'm not sure that the above post is entirely accurate. A major cause of the demise of the Republic was the cost and manpower involved in the Punic Wars with Carthage. Although Rome was successful in both wars, the expense and aftermath were dramatic.
The soldiers who fought in the Punic Wars were citizen soldiers who abandoned their farms to fight. When they returned, their farms had deteriorated from neglect to such an extent that they had no way of restoring them to productivity. In desperation, they sold their farms to a number of wealthy individuals who combined all of the farms into large plantation-like units known as latifundia.The latifundia had the advantage of economies of scale, and soon were able to reduce prices below a level which smaller farmers could sustain. The smaller farmers were then forced to mortgage or sell their lands which were also swallowed up by the owners of the latifundia. Those who sold their farms ended up homeless on the streets; plus since only landowners could vote, they lost their rights as citizens.
The result was increasing class tension to the extent that civil war was threatened. When the Gracchus Brothers (Tiberius and Gaius) attempted to institute reforms that would limit the size of the latifundia, they were assassinated.
Roman soldiers, with no source of income, were often paid by and remained loyal to their commanding officers who often acted in their own interests rather than that of the people. Eventually, a full scale civil war erupted which only ended with the institution of the First Triumvirate.
One of the major causes of unrest and problems during the late republic had roots in the military campaigning and the way that the spoils of war were divided. Once it became established practice for soldiers to be rewarded with land and goods from the places they captured or subdued, they became far more loyal to their generals than to the republic which gave the army and individual generals significantly more political power and left the representatives in Rome afraid of that power.
The results of this shift in loyalty were evident when Sulla marched on Rome and defeated an army that was loyal to the Senate.
There were also troubles within the republic that were caused by feelings of unfairness in terms of the benefits of all the military campaigning going to a few small groups of people and yet the risk was being assumed by the republic as a whole as their young men were marched off to fight but not made citizens or given lands and rights that natural born Roman citizens were given.
These two factors led in large part to the two civil wars and eventually to the downfall of the Republic and the formation of the Empire.