In the last few years, there has been a long push by republicans to change the the perception of the 2001 tax cuts. Why? They didn't work.
They didn't stimulate the economy and they pushed the US budget into deficit, and they didn't really return that much money to people that would actually use the money. They didn't work.
So, there has been this meme, this idea, brought into both mainstream and nutcase republican talking points, that these tax cuts were not the 'right kind' of tax cuts.
The claim is that these tax cuts were not Supply Side tax cuts. This is demonstrably false.
All tax cuts fall into the definition of 'Supply Side'. It cannot be avoided. It is in the very nature of the philosophy of Supply Side economics. One of the main tenants of supply side economics is that tax incentives matter, more than almost any other economic incentive. In fact, most textbooks describe Supply Side economics by their emphasis on cutting tax rates, or changing the tax code. One could make a reasonable claim that this fascination with tax cuts and incentives is all of supply side economics.
This is as you would expect, as the only policy change that Supply Siders consistently advocate is cutting taxes. Well, they do try to advocate others, but the other policy changes they want are tin-foil hat worthy, so they are politely ignored by most mainstream commenters. A call for a return to the gold standard doesn't go over well on national TV.
The basic claim is that lowering the tax rate will incentivize people to work harder. Is this true? If you lower tax rates from 90% to 30%, yes, this is a true statement. But what if you lower rates from 39% to 36%? Well, if you are nice you might call the supporting evidence underwhelming. If you are less inclined to civility, you'll use the word bullshit.
Quoting Bodie, Kane, Marcus (all 20 top MBA schools use this book, so it at the very least is an accurate description of Supply side economics):
"The goal is to create an environment in which workers and owners of capital have the maximum incentive and ability to produce and develop goods."
"Supply siders focus on incentives and marginal tax rates."
I will argue that any broadly based tax cut fits the criteria laid out by the first statement. The 2001 tax cuts were broadly based - essentially every person who made money in 2001 was at least slightly effected. And then, the 2001 had a large positive incentive to owners of capital. The largest part of the 2001 tax cuts went to the highest earners.
I will argue that any tax cut where the richest in our economy, who are largely the owners of capital, get substantial tax breaks of any kind is a supply side cut.
Sometimes you might hear a quote from some famous supply-sider, that these cuts didn't fit their strict definition of what a supply-side tax cut is supposed to be. Don't believe them. They've made their case. You can't make claims over decades, and even campaign on how stimulative any and all tax cuts are for the economy and then run away when they actually come, because they are going to come during a natural downturn in the business cycle.
They have to endure a real world refutation of the theory.