Donald Trump will meet Mexican President Enrique Pena Nieto in an unexpected trip to a country which the Republican presidential candidate has frequently vilified as the main source of illegal immigration and drug smuggling to the United States.
Trump's visit to Mexico City takes place hours before he is due to deliver a highly anticipated speech in the border state of Arizona on how he would tackle illegal immigration if he wins the election.
True to Trump's flair for the dramatic, the trip south of the border will guarantee news coverage. But it also takes him to a country where he is widely disliked because of his harsh comments.
Mexican opposition politicians and academics expressed disbelief that Pena Nieto would host Trump, who has accused Mexico of sending criminals and rapists across the border and cheating the US on trade.
Republican vice presidential candidate Mike Pence said Trump would speak to the Mexican leader about border security, including his signature pledge to build a wall along the border.
Pena Nieto has dismissed Trump's demand Mexico pay for the proposed border wall and likened his tone to the ascent of dictators like Adolf Hitler and Benito Mussolini.
A source close to the Trump campaign said he expects the candidate might apologise in Mexico for some of his rhetoric but stay firm on illegal immigration.
Trump trails Democratic rival Hillary Clinton in most opinion polls nationally and in most battleground states with 10 weeks to go until the election.
Speaking to military veterans in Ohio, Clinton said Trump could not paper over his previous harsh language against Mexico, which helped him defeat 16 rivals for the Republican presidential nomination.
"It certainly takes more than trying to make up for more than a year of insults and insinuations by dropping in on our neighbours for a few hours and then flying home again," she told a convention.
Trump has been pilloried in Mexico since he launched his White House campaign last year.
He has pledged to renegotiate or scrap the 1994 trade deal between the United States, Mexico and Canada.
Former Mexican President Vicente Fox blasted the visit, saying Trump was untrustworthy and that Pena Nieto's invitation appeared aimed at boosting his own flagging popularity at home.
Pena Nieto has been enmeshed in his own controversies, including over whether he plagiarised some of his 1991 undergraduate law thesis.
Afterwards the visit Trump travels to Arizona for a speech in which aides said he would reaffirm his determination to build a border wall and to quickly deport illegal immigrants who have committed crimes.
But the central question was how he would treat the majority of the 11 million immigrants who have set down roots in their communities and obeyed US laws.
Trump has shown signs of indecision on whether to go ahead with his previous proposal for a "deportation force", saying there are some "great people" among the immigrant population and that he would like to work with them.