The most regressive tax is social security. And that one will never be removed, either by Republicans or Democrats.
Sales tax is also regressive, but at least it hits all income levels equally, at least for the things which qualify for sales tax. Higher tax brackets are not hit quite so hard by sales tax, simply because many things that rich folks buy, are not subject to sales tax.
One has to be self-employed to fully understand the burden of the Social Security tax. I'm not a high-roller. Have been self-employed over three decades. Sometimes the Social Security tax bill was higher than the income tax. The two add together of course. Add em togther and send Uncle a painful check. Tax refund? Self-Employed never get tax refunds unless they stupidly overestimate quarterly payments.
Have been 'lucky' because there was never a year so low I didn't owe income tax. Income has gotta get painfully low before Uncle Sugar doesn't charge income tax. But hardly any income is low enough to avoid Social Security.
The guy who nets $1000 a year mowing lawns, or the guy who nets $100,000 a year consulting-- In 2008 both of those guys have to pay Uncle Sugar 15.30% off the net, with no credits or deductions. But any feller who earns more than $102,000, doesn't have to pay SS tax any higher than the $102,000 mark. That is what makes it a 'poor and middle class only' tax.
The non-self-employed, only pay half that, and the employer pays the other half. Any half-way decent employer could painlessly give you a 7.15% pay raise if Social Security tax was abolished.
The standard argument is that this is fair, because it is a 'retirement program'. However, all the SS receipts go into the general fund and are spent as soon as they come in. SS payout system has always been a 'welfare program for old folks'. I think a welfare program for old and disabled makes perfect sense. Its only right.
But it ain't a retirement program. If it was a retirement program, if I die before retirement, the govt would cut my wife a check for all my SS payments + interest.
The FIRST thing we could do to make the tax system fair, is slightly adjust income tax rates (for revenue neutrality), and eliminate the Social Security tax.
NOBODY, EVER, from either party, ever suggests that. It would remove too huge a cash slush fund, paid almost entirely by the poor and middle class.