John Thune needs to think long and hard about whether Senate Republicans want to line up behind the “revenge tax” provision in the House GOP’s budget bill.

The measure’s morphed into a veritable obsession for Wall Street strategists and portfolio managers, who worry the Trump administration’s on the brink of implementing de facto capital controls.

It’s hard to decide whether everyone (present company included) is blowing this out of proportion. A spokesman for the House Ways and Means Committee reiterated this week that an exception for portfolio interest — and all the stipulations associated with that exception — would remain in place, ostensibly exempting income from Treasurys and thereby allaying the market’s worst fears.

The same exemption would presumably cover other sorts of interest, potentially shielding investors who hold, for example, debt issued by US corporations, from the tax as well.

But because that exemption would severely limit the potential income raised from the proposed tax, it’s not clear why bother. That is: If you’re not going to generate enough revenue to matter (i.e., not enough to offset any meaningful portion of the red ink you’re spilling all over the federal ledger), why risk spooking markets by implementing a measure that looks, to some observers, like capital controls?

One answer is that it’s just another form of leverage Trump can use when negotiating with trade partners. That’s fine — I guess — but in his quest for leverage, Trump’s resorting to implicit threats which, if carried through, would have quasi-existential consequences.

For example, failing to defend the Baltic states against hypothetical Russian aggression (rightly or wrongly, some interpreted Trump’s approach to Ukraine as a nod to the dispensability of Estonia, Latvia and Lithuania) could be the beginning of the end for humanity. That’s just barely an exaggeration. The same’s true of allusions to pulling US troops off the Korean Peninsula.

The consequences of US capital controls aren’t existential for the species, but they are for markets. Or could be, anyway. Republicans would vociferously dispute the capital controls characterization of Section 899 — or at least I hope they would. But investors are applying the duck test: “If it looks like a duck, swims like a duck and quacks like a duck, then it probably is a duck.”

Even if key exemptions remain in place (or are put in place), passing the legislation with the “revenge” provision largely intact sends a message: Foreign investors, large, small and in-between, may be subject to a surcharge on some US asset holdings depending on whether local legislation’s deemed “discriminatory” by Washington.

That’s extremely problematic for any number of reasons, not least of which is that in democratic locales, it means foreign investors who voted against politicians who effectuate policy the US doesn’t like will be fined for local policy they didn’t support at the ballot box.

Additionally, and to the extent the executive branch in the US has latitude to determine what’s “discriminatory” and what isn’t, foreign investors will be unable to discern, from administration to administration, whether they’ll have to pay the surcharge. Like this: “Well, my country’s digital tax is ‘discriminatory’ to Trump, but maybe it won’t be to a Democrat, so should I sell my US assets now and then buy them back later?”

You don’t want that. You don’t want fickle money that comes and goes depending on The White House’s subjective interpretation of foreign tax laws. Rather, you want “sticky” money. Indeed, that’s part and parcel of Trump’s whole shtick: He’s going to encourage massive, long-term investment in the US. This idea of his runs entirely contrary to that.

Needless to say, a lot of investors — particularly large ones — would opt to simply avoid US assets subject to the surcharge rather than try to incorporate increasingly mercurial, openly vindictive US tax laws into their decision calculus even if they believe a majority of their holdings will qualify for an exemption.

Ultimately, going ahead with the revenge tax is likely to create more problems than it solves, and that’s if it solves any. If you’re a foreign head of state and/or a foreign legislature and you pass a law, chances are it has some popular support. The notion that Trump’s going to bully those leaders and legislatures into repealing their own laws by threatening to tax private citizens and non-state investors abroad seems far-fetched. And also a bit silly — like going to a foreign capital, grabbing a random passerby and saying, “Repeal your digital services levy or I’ll steal this person’s wallet.”

If you’re a foreigner and you don’t want to be a part of that extortion equation, there are two ways to avoid it. One’s to become American, but unless you’re a Scandinavian supermodel (or Canada or Greenland), Trump doesn’t want you, so that’s out. The other option’s to just stay away from US assets, and maybe even sell some of the ones you already have.

In an interview with Bloomberg on Tuesday, Allianz CIO Ludovic Subran called the 899 threat a “big scary” provision which, if implemented in full, “could spook markets vividly.” In the same interview, Subran branded the revenge tax a kind of “capital control.” If it looks like a duck.