Disregarding all objections to Capitalism for the moment:
Supply-side economics, popular in the Republican camp, is the main rival of Keynesian economics. Supply-siders feel that if production of goods and services is stimulated, demand will increase, thus stimulating the economy. Basically, they propose to do so by cutting taxes. Contrary to what some claim, the main beneficiaries of these tax cuts will be the rich, who supposedly will shovel more cash into business, creating jobs (and thus benefits will "trickle down" to the masses. As if capital gains tax cuts will benefit your immigrant laborer.) However, cutting taxes demands fiscal austerity and spending cuts; something Reagan chose to ignore. It was rather foolish to cut taxes and throw gobs of cash at the Pentagon. This is why we are saddled with a massive debt.
Furthermore, the gap between the rich and poor only widened as the result of Reaganomics; "trickle down" is just that. A trickle. And, as we have seen, much of the upper class is wont to spend tax savings on new yachts and houses, rather than invest in industry.