Mexico’s president Andrés Manuel López Obrador has written to his “friend” Donald Trump warning that any attempt to close the US-Mexico border if he is re-elected would spark a “rebellion” on both sides. Trump made the pledge during his speech at the Republican national convention, where he spoke about the “massive invasion at our southern border that spread misery, crime, poverty, disease and destruction to communities all across our land”. López Obrador’s letter emphasised the economic integration between the two countries and the damage to “people, industry and commerce” that closing the border would bring. California, Arizona, New Mexico and Texas, together with the six border states in Mexico, “represent the fourth biggest economy in the world”, while 1 million people and 300,000 vehicles cross the border every day, wrote the Mexican leader popularly known as Amlo. Many of those vehicles carry cargo as part of the USMCA free trade agreement among the US, Mexico and Canada, which López Obrador defended as making goods cheaper for US consumers and being “the only way to successfully confront the competition brought by the economic and commercial advance of China”. López Obrador also asked Trump to bear in mind that almost 40 million Mexicans live in the US, that seven of every 10 agricultural workers in the US are Mexican, and that Mexicans contributed $325bn to the US economy last year. But, he added: “I understand that you are campaigning and that you are not – as some believe – obstinate”. The letter is not the first the veteran leftist has sent to Trump, with whom he struck an unlikely friendship on coming to power. While on the campaign trail in 2017, López Obrador wrote a book called Listen Up, Trump, in which he compared Trump’s comments about Mexicans to the ways Nazis talked about Jews. Yet the two confounded expectations when they overlapped as presidents from 2018 to 2020, developing an outwardly friendly relationship as two plain-speaking nationalists at the head of anti-establishment movements, despite their apparent ideological differences. Behind the belligerent rhetoric, both were happy to have a transactional relationship on issues such as migration, largely staying out of each other’s domestic issues. As Mexican presidents can only serve one six-year term, López Obrador will leave power on 1 October. Mexico’s incoming president, Claudia Sheinbaum, is a close political ally of López Obrador and has promised to continue his policies, but is a very different figure. Where López Obrador is a folksy populist from the southern state of Tabasco, Sheinbaum is a climate scientist from the intellectual circle of Mexico City. If Trump is re-elected as president, Sheinbaum will be his counterpart in Mexico for the duration of his term – but it seems unlikely she will have the same personal rapport with Trump that López Obrador did. Aside from promising to close the border, Trump has repeatedly threatened the possibility of military strikes on organised crime groups in Mexican territory if he is re-elected. Speaking on Fox News on Tuesday, Trump said cartels were killing 300,000 people a year with fentanyl – the true figure of fentanyl deaths in the US is closer to 70,000 – and that they could “take out a [Mexican] president in two minutes” if they wanted to. JD Vance, the Republican candidate for vice-president, added that Mexico was at risk of becoming a “narco-state” if action was not taken. Asked whether the option of military action within the territory of the US’s biggest trading partner was still on the table, Trump responded: “Absolutely.”