> Supply side is a valid economic theory.
I presume that actually think that this is the case and that you didn't just forget the /s.
Supply-side economics has been thoroughly debunked, again and again[0]. Just look the current state of the US.
> Give people more money and they spend it.
It depends. It depends how much money they have, how much money they already have and what your definition of 'spending' is. Not every kind of 'spending' is of economic and social value. Give a billionaire a dollar and they'll forget it with the lint in their pocket, will get thrown out with the trousers and incinerated along with the trash. Give a billionaire a million dollars and they'll put it into NFT or crypto or something else of questionable social or economical value. It's only when you give a poor person some money that they will spend it in a 'useful' way that actually increases productivity.
But "Give people more money and they spend it" not supply-side economics, actually, is it?
In all actual actuality, supply-side economics is about lowering taxes, decreasing regulation, and allowing free trade, with a heavy emphasis on lowering taxes. You could say that it's primarily about lowering taxes, because 'decrease regulation' and 'promote free trade' is not exactly a supply-side original. SSE is about lowering taxes, primarily for the 'job creators'. That's what the Laffer curve is for. And only the beginning of the issues with supply-side economics ...
> Apparently we are still talking about it despite the economic expansion of the 1980’s
What good is economic expansion if no one but a very small and exclusive group of very exclusive and already very rich people profits said economic expansion? Do larger number make you happy? if so, here's a big number for you: 10↑10.
If you are not more happy now, then you are in the same boat as most US workers since 1979. Just because there was economic expansion, it doesn't mean their lives got better. Worse, in fact. Wages stalled, living expenses increased, social support networks were cut and cut and cut again. Why exactly? Yes, exactly. Supply-side economics, or "voodoo economic policy", as George H. W. Bush famously described it. Presumably because it thrives on the corpses of exsanguinated workers.
> a Democratic ploy [...]
To my knowledge, one of the mayor reasons for the '70ies economic downturn was the "Nixon shock" where prices and wages were frozen and import taxed increased. It's named after Richard Nixon. He was a Republicant President[1].
[0] https://en.wikipedia.org/wiki/Kansas_experiment
[1] https://en.wikipedia.org/wiki/1973%E2%80%931975_recession