MEXICO CITY (Reuters) – Mexican President Andres Manuel Lopez Obrador has received an official list of U.S. products that could be subject to retaliatory tariffs if across-the-board duties threatened by the Trump administration take effect, officials said on Wednesday.

U.S. President Donald Trump has pledged to apply a first round of tariffs on all Mexican imports next week if Lopez Obrador’s government does not stem the flow of mostly Central American migrants seeking entry into the United States.

Four sources familiar with the situation, who spoke on condition of anonymity due to the sensitivity of the matter, said the list was with Lopez Obrador’s office. One source said the president’s office had not taken a decision on the list.

The products targeted are similar to those lined up in retaliation to Trump’s steel and aluminium tariffs last year, and are principally tailored towards hitting the U.S. president’s electoral base, according to one of the sources.

That meant focusing on states that voted for Trump in 2016 where agriculture plays a major role in the local economy, as well as several so-called Rust Belt industrial states, including Michigan, Pennsylvania and Ohio, the source added.

The list submitted to the president’s office excludes U.S. corn, two of the sources said. That could change in due course, one of the sources noted.

Mexico’s growing livestock industry relies on millions of tonnes of U.S.-grown yellow corn annually and industry experts say it would extremely hard to quickly substitute the American imports with corn from other nations.

While Mexico is self-sufficient in production of white corn, used for the country’s staple tortillas, it has for decades relied on yellow corn imports to feed its cows, pigs and chickens.