Donald Trump at Center Stage in Mexican Presidential Debate

U.S. President Donald Trump served as a punching bag in a late Sunday debate among the four leading contenders in Mexico's July 1 presidential election.

Leftist front-runner Andres Manuel Lopez Obrador, from the MORENA party, faced off against his three rivals in a debate set in the far northern border city of Tijuana that focused on international affairs.

As expected, the candidates took shots at Trump, whose often incendiary anti-immigrant rhetoric and vows to build a wall on the southern U.S. border have riled up the Mexican electorate.

Lopez Obrador, who surveys show is 20 points ahead of his nearest rival -- center-right coalition candidate Ricardo Anaya -- said that if elected he would seek "a friendly" relationship with Trump, "but he will have to respect us."

Anaya attacked President Enrique Pena Nieto's decision to invite Trump to Mexico City during the 2016 presidential campaign, which helped boost the real estate magnate's foreign policy credibility.

All four candidates -- including Jose Antonio Meade, third in the polls, with the ruling Institutional Revolutionary Party (PRI) and independent Jaime Rodriguez Calderon -- agreed that Mexico should help Central Americans crossing the country on the way to the U.S. border.

The next Mexican presidential debate is scheduled for June 12.