Both flat tax and sales tax disproportionately tax the poor.

The very rich spend less money relative to how much they make, which puts higher burden on the poor than with our progressive tax.

Flat fax is bad, because 40% of the guy making 20k per year is going to severely change quality of like. 40% from the guy making $40 million has much less impact on his quality of life. This is the whole reason for having a progressive tax... Impact of money on quality of life doesn't work on a linear scale, so neither should our taxes.

I've noticed an increasing number of friends who are very left leaning, promoting the Fair Tax lately.

Most of the issue with support for Fair Tax is that it's enough of an overhaul that people believe it to be an unrealistic pipe dream, which is may well be.

One of the big perks of the Fair Tax is that it makes it easier for people to pay off debt and would actually encourage it. That gets a lot of support from many different angles.

Just to be clear however, the Fair Tax and a Flat tax are very different things that would have wildly different consequences to different people.