The Anthenian Greeks invented democracy, but theirs lasted less than 200 years. As critics of the time, including Thucydides and Aristophanes, pointed out, it was highly subject to sudden and poorly-reasoned decisions — the majority was easily swayed by the passions of the moment or by great orators. These decisions included killing off some of the nation’s own leaders and generals when they disappointed, and committing the country to battles that over-extended its capabilities. It was, essentially, mob rule. The Founders of the United States correctly took that failure as a warning, and restrained the US democracy with the Constitution, which created protections against mob rule, including the diffusion of power across the states and within the government itself as well as by adding the Bill of Rights.
There is a push now on the part of the left to move the country more in the direction of pure democracy, to remove some of the diffusion of power (1,2,3) and to undermine the protections of the Bill of Rights (1,2,3). These two videos from the Atlas Society and PragerU are good illustrations of why we have the Constitution and it’s well-engineered limitations on democracy in the first place.