Claudia Sheinbaum, the president of Mexico, stood underneath a giant Mexican flag and before troops at a military installation in Mexico City. It was Flag Day last month and she used her speech as an opportunity to, figuratively and literally, rally around it.

“Mexico must be respected,” she said, adding later: “Its people are brave. We know that when our people unite around their history, their country and their flag, there is no force in the world that can break their spirit.”

Times had changed, she said: Mexico would not bow down to foreign governments.

Given the circumstances — President Trump’s steep tariffs against Mexico went into effect in the first minutes of Tuesday — Ms. Sheinbaum’s optics were fitting. As Mr. Trump once again targeted Mexico, using the hammer of tariffs as a negotiating tool, a sense of Mexican nationalism has been strengthened.

The Mexican government and businesses have rekindled a “Made in Mexico” campaign. Some Mexicans have called for boycotts of U.S. companies and products, while others have put together lists of Mexican stores and brands to support instead of American ones.

Ms. Sheinbaum is frequently featured on the front page of local newspapers with members of the country’s military or in front of a giant Mexican flag. Private companies have taken out nationalistic advertisements, one featuring the president leading the masses and carrying a banner saying, “Mexico united, never defeated!”

And Ms. Sheinbaum, who has been trying to balance a pro-Mexico drumbeat while advocating cooperative dialogue with American officials, has seen her approval ratings rise as high as 80 percent, according to one poll. She has not only succeeded a popular president, Andres Manuel López Obrador, who reshaped Mexican politics and was her mentor, but has come into her own at a time of global upheaval under Mr. Trump.

“There’s a lot of support for the president now,” said Juan Manuel Sánchez, 57, an artisan in Mexico City who also praised Ms. Sheinbaum’s crackdown on drug trafficking.

During his first term, Mr. Trump used tariffs to renegotiate the North American Free Trade Agreement and strike a new U.S.-Mexico-Canada agreement, which he signed in 2020. He has used similar tactics now against Mexico and Canada, while arguing that too many illegal drugs and migrants are flowing from the two countries into the United States.

A month ago, Mr. Trump signed an executive order calling for 25 percent tariffs on Mexican imports. But less than a day before they were to go into effect, Mr. Trump and Ms. Sheinbaum spoke on the phone and announced an agreement to delay them for 30 days.

Under the terms of that deal, Mexico posted an additional 10,000 Mexican National Guards troops on the border to help stem the flow of fentanyl and migrants into the United States. In return, Ms. Sheinbaum said, the U.S. government would work to stop the flow of guns into Mexico.

Even though the number of migrant crossings at the southern border has dropped to once unthinkable levels since Mr. Trump took office in January, Mexican officials were significantly deterring migration to the United States months before. Last week, Mexico sent nearly 30 top cartel operatives wanted by American authorities to the United States, one of the largest such handovers in the history of the drug war.

“There’s a lot of unity in the country in the face of what is happening,” including Mr. Trump’s economic threats, Ms. Sheinbaum said on Monday, hours before the tariffs took effect.

Although Mr. Trump insisted on Monday that the tariffs would begin the next day, the cloud over Mexico from the north has loomed since his most recent presidential campaign. It led to uncertainty and frustration but also boosted national pride.

Agustin Barrios Gómez, a former Mexican congressman and a founding member of the nonprofit Mexican Council on Foreign Relations, said that even Mexicans who didn’t vote for Ms. Sheinbaum “understand that right now, Mexico’s national interest — beyond party politics — is to rally around our president.”

One reason for the surge in support for her, Mr. Barrios Gómez said, was to ensure Ms. Sheinbaum has enough political capital within the country to be in a stronger negotiating position with Mr. Trump come what may.

Nationalism is complicated in Mexico, Mr. Barrios Gómez said, because it is so intricately intertwined with the United States geographically, culturally and economically, as well as with immigration and security.

“We are not neighbors, we’re roommates,” he said. In other words, analysts said, the U.S. tariffs against Mexico will hurt both economies, as would the reciprocal tariffs suggested by Ms. Sheinbaum. (Mr. Trump is also threatening separate 25 percent tariffs on global steel and aluminum imports, which would affect Mexico.)

For Mexico, the tipping point against the United States has not been reached, Mr. Barrios Gómez said late last week before the tariffs went into effect, but “if you call someone your enemy enough, you might just turn them into one.”

The specter of a trade war between the countries has changed the perception in Mexico of Mr. Trump and of its relationship with the United States.

According to the Mexican polling film Buendía & Marquéz, the number of respondents in Mexico who believed the relationship between Ms. Sheibaum and Mr. Trump was at least good dropped significantly between last November and February, while the number of respondents who have a negative opinion of Mr. Trump jumped to 80 percent in mid-February from 66 percent in early January.

Mr. Trump has nevertheless praised Ms. Sheinbaum as a “marvelous woman” while mocking Canada’s prime minister, Justin Trudeau. Mr. Trudeau, who has become increasingly unpopular at home, is entering his final days in office while Ms. Sheinbaum’s popular foundation is stronger. She resoundingly won election last summer and began her six-year term in October.

During her Monday morning news conference, Ms. Sheinbaum once again called for calm ahead of Mr. Trump’s tariffs deadline and said she hoped to strike a last-minute deal, which did not materialize. “Obviously we don’t want there to be tariffs,” she said, adding that her government would respond.

Since before Mr. Trump’s inauguration, her administration has been promoting what it calls “Plan Mexico,” a strategy meant to diversify its economy to make it less dependent on the United States, to reinvigorate Mexican manufacturing and propel the country to become one of the world’s top 10 economies. (It is currently the 15th largest, according to the International Monetary Fund.)

As part of that effort, Ms. Sheinbaum’s administration started the “Made in Mexico” drive, in which an official seal is placed on products made in the country that meet certain requirements. The seal, with an illustration of a Mexican eagle, was created in 1978 to promote Mexican goods and has been revived by presidents over the years.

As the threatened U.S. tariffs were paused a month ago, Mexico’s secretary of economy, Marcelo Ebrard, told companies that the government wanted to once again push the “Made in Mexico” seal.

Last week, Walmart Mexico, the largest private employer in the country with 200,000 workers, unveiled its efforts to put the “Made in Mexico” seal — with the added word “proudly” — in the aisles of its 3,000 stores throughout the country. Although Walmart is an American brand, Javier Treviño, Walmart Mexico’s senior vice president of corporate affairs, said the company wanted to show customers that it is a Mexican entity and that most of the products it sells are made within the nation.

The campaign “is very important for us because we have to strengthen investment and confidence in Mexico and ensure that the economy can grow, because the environment is not easy,” Mr. Treviño, a former Mexican congressman, said in an interview.

Other big companies have joined Ms. Sheinbaum’s push, including Grupo Modelo, the brewing giant that makes Corona and Modelo beers, which announced it would put new “Made in Mexico” caps on bottles.

On Saturday, Mr. Sánchez, the Mexico City artisan, was at his neighborhood market, which, he said, proved that he prefers to shop locally. Before Mr. Trump’s tariffs went into effect, he said he might consider boycotting U.S. companies and products if they did.

Unlike in Canada, where locals have been shunning American products and buying more Canadian flags since Mr. Trump threatened the tariffs, Mr. Sánchez said that Mexicans were already nationalistic and that most had a flag.

“But when something very serious happens here,” he said, “we all unite.”

Maria Abi-Habib contributed reporting from Mexico City.