If Donald Trump is elected president and implements a promised massive shakeup of the Washington establishment, historians might pinpoint Aug. 31, 2016, as the day the world really took notice.

That's because he accepted an invitation from the president of Mexico to visit, prompting politicians on both sides of the border to erupt with antagonism.

On the U.S. side, Democratic candidate Hillary Clinton's campaign trotted out comments Trump has made about Mexico-U.S. issues, such as the porous border that allows Mexicans to enter illegally, the drug cartels that operate with impunity on the Mexican side and more.

"From the first days of his campaign, Donald Trump has painted Mexicans as 'rapists' and criminals and has promised to deport 16 million people, including children and U.S. citizens. He has said we should force Mexico to pay for his giant border wall. He has said we should ban remittances to families in Mexico if Mexico doesn't pay up. What ultimately matters is what Donald Trump says to voters in Arizona, not Mexico, and whether he remains committed to the splitting up of families and deportation of millions," said a statement from Jennifer Palmieri, a spokeswoman for Hillary for America.

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And from south of the border came the words of outraged former President Vicente Fox, who called the visit political exploitation.

"It's a very opportunistic move, and I hope U.S. public opinion, U.S. citizens, can see this and finally, and finally see what is behind Trump, the false prophet that is just cheating everybody," Fox said during a Skype chat, Politico reported.

The visit Wednesday was at the invitation of Mexican President Enrique Pena Nieto. To Trump supporters, it was a display of his presidential capabilities – the willingness to meet with someone with whom he's had significant disagreements.

Gov. Mike Pence, Trump's running mate, said that the visit is simply a glimpse of Trump's abilities as president.

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"I don't know where Hillary Clinton is. I heard she's going to Cincinnati today, but I think the American people can see quite a contrast between Donald Trump – who gets an invitation from a world leader, drops what he's doing even with a major speech tonight – and heads down to Mexico to sit down with the president of that country to begin a conversation about how we move the interests of the American people forward and how we work with our neighbors in a more constructive and effective way," Pence said.

He said it's an opportunity to see what Trump's leadership is like.

"The opportunity to sit down with our neighbors to the south and to have these two leaders begin a relationship that will be a foundation for negotiations I think should be a real insight to millions of Americans who are getting a glimpse here of the kind of president that Donald Trump will be," he continued.

In contrast, former Mexican first lady Margarita Zavala de Calderon said on social media, Trump is "not welcome."

"Mexicans have dignity and repudiate his hate speech," she claimed.

Much of the rancor is over Trump's desire to build a security wall along the Mexican border and have Mexico pay for it as a first step in tightening security, eliminating a flood of illegal aliens who are costing American taxpayers billions in health care, education, housing and other assistance.

Former Mexican President Felipe Calderon, who has vowed his country wouldn't pay a cent, retweeted the message from his wife, and Mexican-born Univision broadcaster Jorge Ramos said, "In simple terms, today's meeting is between 2 of the most despised and hated people by millions of Mexicans on both sides of the border."

Other criticism:

Roberto Gil Zuarth, the senate president in Mexico, said the meeting only legitimized Trump's "proposal of demagogy and hate."

Guadalajara Mayor Enrique Alfaro Ramirez blamed Nieto for issuing the invitation: "A person who openly incites hatred, racism, violence and even war, should not be invited into our country. After treating Mexicans like he has, his visit offends our dignity."

Former Mexican ambassador to the U.S. Miguel Basanez wrote, "I deeply regret the invitation."

"I want to hide," wrote Jorge Guajardo, a former Mexico consul in Austin, Texas.

Politico's Kyle Cheney and Ben Schreckinger pointed out the trip could be a gamble for Trump.

"If he can go down there and look statesmanlike while at the same time being firm for what he stands for, then I think it helps shape people's perceptions of him as to what he would be like if he's actually president," Steve Munister, former chairman of the Texas GOP, told Politico.

"This is a chance for him to demonstrate to the American people what it would be like to have Trump as president, what it would be like if he was dealing with foreign leaders."

Politico noted the hurdles, however, since Trump last year described Mexico as "not a … friend," based on its policies and actions.

Nieto had invited both Trump and Clinton to visit. Clinton's campaign said its candidate would meet with Nieto later.

Buzzfeed reported the meeting would be an important test of Trump's ability to sway foreign leaders.

Meanwhile, at least part of the Clinton campaign's frustration is that Trump is meeting first with the leader of America's neighbor to the south

She repeatedly has touted her international experiences as secretary of state.

The meeting, focusing on trade, security and immigration, came just as a USC/Los Angeles Times daily poll showed 45.3 percent support for Trump and 41.9 percent for Clinton.

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