The plan already has critics who argue the change is outside of the Treasury's authority and it's likely to be challenged in court.

And even if the proposal succeeds, it wouldn't reach many wallets, said Daniel Hemel, assistant professor at The University of Chicago Law School.

"For ordinary investors, it won't make much of a difference," Hemel said. "Their primary investments are in their 401(k), individual retirement accounts and life insurance, and they're not going to pay capital gains tax there anyway."

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