Leftist front-runner Andres Manuel Lopez Obrador of the National Regeneration Movement (MORENA) speaks to supporters during a campaign rally in Uruapan, in Michoacan state, Mexico, June 8, 2018. REUTERS/Alan Ortega

MEXICO CITY (Reuters) - A spate of calls in recent days attacking Mexico’s leftist presidential frontrunner has put his supporters on edge and underscored the last-ditch attempts to dent the candidate’s commanding lead three weeks before the election.

This week, Mexicans have stormed social media sites to complain about the automated calls targeting Andres Manuel Lopez Obrador, who has a double-digit lead in most polls ahead of the July 1 vote.

Mexico’s National Electoral Institute (INE) and specialized prosecutor for electoral crimes (FEPADE) said on Thursday that they would investigate the calls.

The calls take aim at Lopez Obrador on various fronts, from his relationship with the nation’s teacher’s union to his plans to explore amnesty for some drug criminals. Some of the calls take the form of polls, asking listeners to mark “1” on their phones if they agree with the proposal.

“Are you for or against pardoning those who committed crimes related to drug trafficking?” a robotic voice asks in one of the calls recorded by Reuters. “(Lopez Obrador) has also said he will withdraw the army and the marines from combating drug trafficking, even though they are the ones who have successfully detained many of the drug trafficking kingpins.”

Speaking at a campaign event on Thursday, Lopez Obrador denounced the calls as part of a “dirty war” being waged against his candidacy.

Mounting his third campaign for the presidency, Lopez Obrador has drawn the ire of many in Mexico’s elite, particularly members of the business community, who oppose his plans to review oil contracts and a new airport for the capital that is already under construction.

But his lead has continued to widen over former finance minister Jose Antonio Meade, the candidate for the ruling Institutional Revolutionary Party (PRI), and Ricardo Anaya, a former chairman of the center-right National Action Party (PAN), who is fronting a right-left coalition.

Nevertheless, the negative campaign tactics have not been limited to Lopez Obrador. On Thursday, a video was published accusing Anaya of laundering money to finance his campaign. Anaya has rejected the accusations and accused President Enrique Pena Nieto of trying to discredit him.