Brazil’s presidential race next year was shaping up to be one Americans might find familiar: an aging incumbent with waning popularity trailing a brazen populist who claimed the last election had been stolen from him.

Then entered President Trump.

Mr. Trump’s threat last week of 50 percent tariffs on Brazilian exports as a way of saving his ally, former president Jair Bolsonaro, from possible imprisonment, has reshuffled Brazil’s political landscape, giving President Luiz Inácio Lula da Silva an unexpected boost.

With Mr. Trump and his politically motivated tariffs as a foil, Mr. Lula suddenly has a clear message: We will not back down to a bully. His stance is drawing praise in the press, going viral online and giving his supporters new hope that Mr. Lula could win a fourth term next year, days before he turns 80.

They have reason to be optimistic: days after Mr. Trump’s tariff threats, Mr. Lula’s approval ratings rose to their highest level in months. New polls showed that 43 to 50 percent of Brazilians approved of his performance, up three to five percentage points since May.

“It was a stroke of luck for the president,” said Camila Rocha, a political scientist at the Brazilian Center for Analysis and Planning, a research institution. “This strengthens him a lot.”

The shift in public opinion is yet another example of the anti-Trump bump, a global phenomenon that has reshaped elections in Canada, Australia and elsewhere by supercharging support for politicians who defy the U.S. president.