In Washington D.C., one of the selling points of an ambitious border-tax plan rests on a key economic assumption: The dollar will appreciate enough to offset any increase in the cost of cheap, imported goods that so many Americans have come to rely on.

It's just Econ 101, backed by well-established macroeconomic theory.

But on Wall Street, traders and strategists who make a living in the $5.1-trillion-a-day currency market say such notions are preposterous. Even if congressional Republicans can set aside their differences to pass the proposed border-adjusted tax — a prospect that seems more remote with each passing day — you'd be hard-pressed to find anyone in the market who believes it will result in the greenback strengthening 25%, as the plan suggests.