While attending events on Sunday where they were selected by their opposing parties as presidential candidates in Mexico’s July 1 election, two politicians pledged to take a tougher position against U.S. President Donald Trump’s proposed border wall.

In recent polls, leftist Andres Manuel Lopez Obrador, 64, of the Morena party holds a double-digit in the race, although right-left coalition leader Ricardo Anaya has recently gained ground.

Nominated on Sunday by the ruling Institutional Revolutionary Party, former finance minister Jose Antonio Meade, 48, trails Lopez Obrador by as much as 20 points, Reuters reported.

In a speech to several hundred Morena supporters gathered at a hotel in Mexico City, Lopez Obrador said that Trump’s proposed wall along the U.S.-Mexico border is unnecessary and will leave problems unresolved.

“If he insists on building the wall, we’re going to turn to the United Nations to defend the rights of Mexicans,” said Lopez Obrador, the former mayor of Mexico City who has run twice for the presidency. “I’m conscious of my historic responsibility.”

Anaya addressed his opposition to the U.S.-Mexico border wall, contending that he would refuse to pay for its construction.

“Mexico will never again be treated like a doormat for the United States, as it’s been in this government,” said Anaya, who is backed by the center-right National Action Party and the center-left Party of the Democratic Revolution.

Key issues in Mexico’s presidential election include addressing crime and impunity, fighting public corruption, raising the minimum wage and improving education to support an economy based on “knowledge” rather than “manufacturing.”