Quote from Sodajerk:
Paradoxically, if this tax went into effect, virtually no one would pay it because the retail trader would go extinct.
Not sure what % of the equity market is retail traders (aka, day traders), but it has to be fairly significant. Taxation of the US equity markets would cause these traders to flee to other markets such as FX. I really haven't closely followed the details of this proposal (since I don't believe it will happen), but if FX, brokered by US firms, fell into this taxation bucket, then traders would find overseas brokers, to avoid this tax.
Net-net, would be a significant loss of liquidity from US traders, a more volatile US stock market due to decreased liquidity, and little additional tax revenue.
Then again, those who can't do serve, and those who can't serve well, serve as policiticians. I'm sure this plan isn't well understood by most of Congress, and politically, as an earlier poster stated, may be favored by the masses. So, who knows, maybe this gets passed, especially if there is some sort of 401K carve-out in the final version.