President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE on Sunday night congratulated the winner of Mexico's presidential election, a left-wing populist who has vowed to take a tougher stance against the U.S. president.

Andrés Manuel López Obrador beat out the candidate representing the ruling PRI party on Sunday amid growing discontent over the country's rampant crime and corruption. A third-time presidential candidate, López Obrador has publicly derided Trump's plans to build a wall along the border between the U.S. and Mexico and has said he will demand respect from the White House.

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Trump, who has had a tense relationship with current Mexican President Enrique Peña Nieto, offered kind words to López Obrador after his resounding victory over PRI candidate Jose Antonio Meade.

"Congratulations to Andres Manuel Lopez Obrador on becoming the next President of Mexico. I look very much forward to working with him," Trump tweeted. "There is much to be done that will benefit both the United States and Mexico!"

Relations between the two countries have taken a dramatic turn for the worse under Trump, due in large part to the president's frequent attacks on immigrants from Central America and his vow to build a wall and have Mexico pay for it.

Trump also announced new tariffs on Mexican imports, which were promptly answered by Mexico with corresponding retaliatory measures on U.S. goods.

Trump meanwhile has frequently railed against the North American Free Trade Agreement (NAFTA), which he has threatened to scrap if Mexico doesn't agree to concessions imposed upon it by the U.S. The two countries, along with Canada, have held multiple rounds of talks on the deal.

The president on Sunday said he would wait until after the midterm elections in November to sign a new NAFTA deal.