They’re studying his interviews, bracing for mass deportations and preparing policy proposals to bring to the negotiating table.

As Mexico heads toward its presidential election next month, government officials and campaign aides are also girding for a different vote: one in the United States that could return Donald Trump to the presidency.

The last time Mr. Trump took office, his win surprised many of America’s allies, and his threat-filled diplomacy forced them to adapt on the go. Now, they have time to anticipate how Mr. Trump’s victory would transform relations that President Biden has tried to normalize — and they’re furiously preparing for an upheaval.

For some, the memory of negotiating with Mr. Trump the last time he was in office, when he used extreme threats against Mexico, looms large.

What it took to reach a deal with Mr. Trump’s team back then? “Time, patience, cold blood,” the former Mexican foreign minister Marcelo Ebrard said in an interview. “You can win, if you understand this. It’s not easy.”