1) Compressing and simplifying the tax code, and thus encouraging the accumulation of private fortune without limit. This in turn encouraged investments in short-turnaround, high-short-term return investments such as financial instruments over the longer-turnaround investments that actually create wealth and employ people.
2) Deregulation of business, particularly the financial industry. Together with the tax changes noted above, this encouraged and allowed the kind of complicated financial shell-games that collapsed in 2008. Before that, though, it encouraged the increased share of the economy taken up by the financial sector, as opposed to the sectors such as manufacturing and services that genuinely produce wealth.
3) Anti-union policies. Together with the encouragement of outsourcing (below), this was responsible for a long-term decline in real wages.
4) Tax and trade policies that encourage outsourcing. This encouraged manufacturers to shed expensive American labor for the kind of cheap labor available in third-world countries. As the manufacturing jobs, which let many working-class people live middle-class lifestyles, were replaced by service jobs, the government's anti-union policies prevented the unionization of the service industry that might have kept wages high in spite of this change.
There are some other minor policies but those were the main ones.
Note that the Clinton administration was in some ways as culpable here as the Reagan and Bush administrations (both Bushes), and that the Obama administration has not reversed any of this to a significant degree. This is not a Democrat/Republican issue, although arguably the Republicans have been worse.