In writing about Donald Trump, there is a daily temptation to say that this time he’s gone too far. Until the Republicans on Capitol Hill gain an ounce of independence and self-respect, that is wishful thinking. As the rogue President has demonstrated many times, he can do or say almost anything, and the lickspittle Republicans won’t venture beyond offering carefully qualified verbal criticisms, if they muster even that. So it’s unrealistic to expect that Trump will suffer any immediate political consequences for his threat to impose tariffs on all goods imported from Mexico unless the Mexican government “substantially stops the illegal inflow of aliens coming through its territory.”

On Thursday evening, Senator Chuck Grassley, the chairman of the Senate Finance Committee, called Trump’s threat a “misuse” of his Presidential powers, pointing out that trade policy and border security are separate issues. Senator Joni Ernst, Grassley’s Iowa colleague, warned that Trump’s move could derail congressional approval of a new trade deal with Canada and Mexico. None of this will concern Trump unduly. He has grown used to Republicans and corporate trade groups carping about his various trade wars, and he knows that there won’t be any meaningful follow-through, as long as his base of supporters and his right-wing-media allies back him up.

The only real constraints on Trump’s actions are the courts, the opinion polls, and the financial markets. On Friday, the U.S. Chamber of Commerce, which represents Fortune 500 companies, like General Motors and Ford, whose businesses are dependent on imports from Mexico, said that it was considering legal action. “We have no choice but to pursue every option available to push back,” Neil Bradley, the organization’s chief policy officer, said.

It’s not clear how far the legal challenge will go. In issuing his latest threat, Trump was relying on the International Emergency Economic Powers Act, of 1977, which gives the President broad authority to respond to an “unusual and extraordinary threat” to the national security, foreign policy, or economy of the United States. Previous Presidents have used this law dozens of times to take action against Russia, Iran, and other countries. But, as CNN’s Zachary Wolf pointed out, the legislation has never been invoked to impose tariffs, and Mexico is an ally, not an adversary. Still, Stephen Vladeck, a law professor at the University of Texas, whom Wolf quoted, said that Trump’s actions “may well be within the letter of the law here.”

If the new tariffs do go into effect, they will raise the prices for consumers on a wide range of goods, which could produce a popular backlash. Recent polls suggest that a majority of Americans already look negatively on Trump’s protectionist trade policies and a plurality dislike his immigration policy. In a Quinnipiac University survey, fifty-three per cent of respondents said that they disapproved of Trump’s handling of trade, and thirty-nine per cent said that they approved. In the latest online poll from The Economist and YouGov, forty-seven per cent of respondents disapproved of Trump’s immigration policies, and forty-four per cent approved.

However, what the White House really cares about is the attitude of Republican voters. As long as they remain solidly behind Trump, most elected Republicans will be, too. According to the Quinnipiac poll, eighty-two per cent of self-identified Republicans approve of Trump’s trade policies. And, in the Economist/YouGov poll, eighty-five per cent of Republicans expressed support for his immigration policies. We’ll have to wait and see how these voters react to a policy that cites the situation at the southern border as a justification for a new tariff, but it seems unlikely that it will prompt Trump’s backers to desert him en masse.

That leaves the stock market, which Trump watches closely, and which could yet prove to be the most effective constraint on his actions. Since the start of May, when the trade talks between the United States and China broke down, and Trump stepped up his threats of expanding his trade war, the Dow Jones averages have fallen more than six per cent. On Friday, after Trump issued his latest threat to Mexico, the Dow tumbled more than three hundred and fifty points, or about 1.4 per cent.

Actually, that was a pretty modest dip compared to major falls in the past, and it suggests that many investors are assuming that the new tariffs won’t go into effect, or they will be quickly reversed. But this could be another example of wishful thinking. The White House says that, if Mexico doesn’t meet its demands, a five-per-cent, across-the-board tariff will be imposed on June 10th—a week from Monday. Subsequently, the rate will be increased to ten per cent, on July 1st, to fifteen per cent, on August 1st, and so on, until it reaches twenty-five per cent. Setting aside the utter paucity of justifications for Trump’s démarche, this is a ridiculously tight timetable, and the President of Mexico, Andrés Manuel López Obrador, has already indicated that he won’t jump to attention. (On Thursday, López Obrador sent Trump a letter in which he said, “Please, remember that I do not lack valor, that I am not a coward nor timorous but rather act according to principles.”)

Based on his experience of the last two years, Trump seems to believe that he can target anybody for his bullying, acting as arbitrarily as he wants, and he still won’t suffer any consequences, because the economy and the stock market will continue to power ahead. This theory is about to be tested. Recent releases indicate that the economy has already slowed in the second quarter of the year, which leaves it more vulnerable to negative shocks, such as the imposition of more tariffs or a big fall in the stock market. Moreover, this latest Trump power play is so extreme and potentially self-destructive that, according to the Wall Street Journal, even his own hard-line trade adviser, Robert Lighthizer, opposed it.

On Wall Street, there is a lot of nervousness about where things are heading, and whether Tariff Man understands the risks that he is taking. Financial markets don’t usually go south gradually; they collapse suddenly, in a heap. “I keep reading [that] Trump views the stock market as a real-time indicator of how he's doing,” Ian Shepherdson, the chief economist at Pantheon Macroeconomics, tweeted on Friday. “Either that’s wrong, or he’s going to have to blink on tariffs, because the market can’t live with this level of crazy.”