Knowing the Dems, it will be the worst case scenario.
That's one of the reasons I just day trade options the last day of the year and don't hold them over, just to avoid dealing with that.
But I day trade them mostly anyway so it's not anything burdensome in the big picture. I feel like anyone wanting to invest longer term is going to be a great deal more shy now before committing, especially if there are cash flow issues.
The other problem is that once the camel has nosed into the tent, them Dems will just make it more and more punitive, like turning it on homeowners.
If you're primarily day-trading, then mark-to-market isn't going to affect you that much, particularly if you don't hold any positions from one tax year to the next.
Where it will hit hard is on people who hold for the long-term, and who do not have a lot of spare cash to pay the tax bill each year on the net MTM gains.
That, of course, implicates most people's savings funds (i.e., investment portfolios outside of things like IRAs and 401(k)s).
The real "tell" will come, of course, when they start making serious noises about taking over IRAs and 401(k)s, and "replacing" them with some sort of supercharged social security benefits (the reason for that being to deal with the just compensation clause of the Constitution - if they can get enough judges to buy the bullshit that some supercharged version of social security is "just compensation" for taking the IRAs and 401(k)s, then they will get all of that value without having to pay for it).