MONTERREY (Reuters) - Mexican left-wing presidential candidate Andres Manuel Lopez Obrador has an 18-point lead ahead of the July 1 election, according to a poll published on Monday that showed him with a growing advantage at the start of formal campaigning.

Leftist front-runner Andres Manuel Lopez Obrador of the National Regeneration Movement (MORENA) waves to supporters during his campaign rally in Ciudad Juarez, Mexico April 1, 2018. REUTERS/Jose Luis Gonzalez

Lopez Obrador, who launched his campaign on Sunday, holds 38 percent of the vote, according to the poll by Parametria, published by Reuters on Monday ahead of wider distribution. That compared to 35 percent in its previous poll.

A Lopez Obrador victory could usher in a Mexican government less accommodating toward the United States, where President Donald Trump has stoked trade tensions with Mexico and aggressively moved to curb immigration.

Lopez Obrador has backed the North American Free Trade Agreement, but his plan to review newly issued oil contracts sparked worries he will deter foreign investment.

One of the factors that appeared to extend his lead was the expulsion of two independent hopefuls from the ballot for not reaching the required number of signatures.

The previous poll is not an exact comparison because it included the now-disqualified candidates.

Neither second-place Ricardo Anaya nor third-place Jose Antonio Meade showed any sign of catching up with former Mexico City mayor Lopez Obrador.

“Lopez Obrador is breaking his ceiling ... he’s growing in a way that wasn’t expected,” Parametria founder Francisco Abundis said.

Anaya, running for the right-left “For Mexico in Front” coalition, holds 20 percent of vote preferences, compared with 21 percent in the previous poll.

Former finance minister Meade, running for the ruling Institutional Revolutionary Party (PRI), stood still on 16 percent of the vote, while independent candidate Margarita Zavala rose to 13 percent of the vote from 10 percent previously.

The youngest candidate, Anaya, 39, launched his campaign on Thursday, pitching himself as a forward-thinking alternative both to the unpopular PRI and Lopez Obrador’s personalized leadership.

Meade’s launch on Sunday highlighted his experience in government under two different political parties, unusual in Mexican politics.

But the focus was mostly on silver-haired Lopez Obrador, who struck a more nationalistic tone in his speech in the border city of Ciudad Juarez, demanding respect from U.S. President Donald Trump.

He also repeated a promise to cancel Mexico City’s new $13 billion airport, the country’s largest infrastructure project.

All major polls show Lopez Obrador far ahead, with most showing his lead growing, although Parametria gives him the widest advantage. One poll in March put Meade in second place, although most show Anaya ahead of him.

Attention has turned to which party will hold sway in the Senate and lower house of Congress, with some believing that Lopez Obrador’s party, the National Regeneration Movement, could win the biggest share in both.

One game-changer could be the first television debate between the candidates on April 22.

Thirteen percent of those polled either did not answer, responded that they did not know or could not choose a candidate listed.

The poll of 800 people had a margin of error of 3.5 percentage points. It was taken face-to-face from March 23-28, before campaigns officially started.