Since March, the son of the former Brazilian president Jair Bolsonaro has been roaming the halls of the White House.

Eduardo Bolsonaro, 41, is a sitting Brazilian congressman, but his latest job has been to convince U.S. officials that a dangerous Brazilian Supreme Court justice wants to throw him and his father into prison, simply for fighting against what they claim was a stolen election.

And in multiple visits to Washington over the past several months, he has found a sympathetic audience.

“This is nothing more, or less, than an attack on a Political Opponent — Something I know much about!” President Trump posted online Monday. “It happened to me, times 10.”

A review of Mr. Trump’s comments about tariffs over the decades shows he has often been fairly vague on the topic, and only more recently came to describe them as the centerpiece of his approach to trade.

Far more frequent and durable has been Mr. Trump’s repeated refrain that other countries are turning the United States into “suckers.” His references to tariffs often came as part of his description of a feeling of national injury that became common as the country’s manufacturing base began eroding. That attentiveness to trade as an issue, even absent a cohesive policy plan, helped Mr. Trump win in 2016.

This year, administration officials say they have brought in roughly $100 billion in tariffs so far. But after pledging that the president would get “90 deals in 90 days,” the White House has secured only a handful of trade agreement frameworks with other countries.

Instead of treating tariffs as one tool that is part of a broader trade strategy, Mr. Trump often describes them as an end unto themselves. While business experts and corporate leaders say the tariffs will raise costs on their products and for consumers who rely on imports, the president has largely dismissed those concerns. And he has relied on his own belief that markets and long-term concerns will eventually level out, even as economic experts have wondered what the end game is.

“What does winning look like? What is the definition of victory?” said Maurice Obstfeld, a senior fellow at the Peterson Institute for International Economics. “He seems to view the definition of victory as eliminating other countries ‘unfair trade practices,’ which in his mind translates one-for-one into reducing our bilateral deficits with every single trading partner.”

Since re-entering office, President Trump has announced a barrage of tariffs to try to rewire the global economy. The trade actions have taken effect in fits and starts, resulting in wild swings in markets and fresh tension among some of America’s closest trading partners.

What’s the latest?

Mr. Trump this week threatened high tariffs on a wide range of targets.

The president said Saturday that many goods from the European Union and Mexico, two of America’s largest trading partners, would be subject to 30 percent tariffs starting next month. E.U. officials had been negotiating with the United States over the past few months in hopes of striking an agreement to avoid such steep levies.

Mr. Trump on Thursday took aim at another major trading partner, threatening a 35 percent tariff on many goods from Canada, in part because of the country’s role in allowing the flow of fentanyl into the United States.

It came a day after he announced a plan to impose a 50 percent tariff on all imports from Brazil, suggesting that the new levies were partly a response to a “witch hunt” against Brazil’s former President Jair Bolsonaro. A political ally of Mr. Trump, Mr. Bolsonaro is facing trial for attempting a coup.

Economists expect the tariffs, which Mr. Trump sees as a way to revive American manufacturing and add revenue to the federal budget, will reduce U.S. imports. American companies, as well as their business partners abroad, would suffer lower profit margins as the government levies are split between consumers, importers and exporters. Firms would be left with less money to pay wages and invest in their growth.

Facing Mr. Trump’s accusations of unfair trade practices, some countries on his Monday mailing list have few bargaining chips. South Korea, for example, 13 years ago inaugurated a free-trade agreement with the United States that has reduced nearly all tariffs between the two countries to zero.

South Korea’s newly elected president, Lee Jae Myung, tried to come up with offers, such as cooperation in helping the United States rebuild its shipbuilding industry. He sent both his top trade negotiator and his national security adviser over the past few days to meet with Trump administration officials to try to hash out a deal.

Nevertheless, South Korea was met with a 25 percent tariff, the same as the one that was paused in April. Although all the new tariffs will not go into effect until Aug. 1, creating a window to de-escalate, it remains unclear what could persuade Mr. Trump to relent.

“We are doing our best to bring about a result mutually beneficial to both sides, but we have been unable to establish what each side exactly wanted from the other side,” Mr. Lee said last Thursday.

One of the many unexpected twists in President Trump’s use of tariff threats came early this month, when he blasted the BRICS, a group of 10 countries that’s named for five of them: Brazil, Russia, India, China and South Africa.

“Any Country aligning themselves with the Anti-American policies of BRICS, will be charged an ADDITIONAL 10% tariff,” he wrote.

Most of the group’s members are closer to China than to the United States, and they have responded to Mr. Trump’s threats with defiance. Brazil’s president, for example, accused him of acting like an “emperor,” one of many barbs exchanged between the two leaders as trade tensions have intensified.

But India — the world’s most populous country, and the “I” that helped make BRICS a word in 2001 — stands apart. As Mr. Trump’s tariffs have reordered global trade, pushing some countries closer to Beijing, India is alone among the founding BRICS members in wanting to reduce China’s sway in world affairs.