MEXICO CITY - Mexico's main opposition presidential candidate Xochitl Galvez called her rival, frontrunner Claudia Sheinbaum, "the candidate of lies" during a pre-election debate on Sunday and dared her to drink a bottle of water from the capital city she governed.

The attack by Galvez, who is representing a three-party opposition coalition, comes as the latest opinion polls show she is struggling to gain momentum with five weeks to go before the June 2 election.

An April 10-13 poll by Mitofsky for newspaper El Economista gave Sheinbaum 51.4% support, with Galvez 26.7% of the vote. Political analysts said the latest debate, which was light on details, was unlikely to make much of an impact on the election's outlook.

"I think all eyes were on Xochitl on this occasion," political analyst Viri Rios told Milenio TV after the debate, saying that Galvez had failed to highlight enough of her own policies during the debate.

"She was confrontational, with the only aim of giving nicknames to Claudia and not necessarily to talk about what she wants to do," Rios added.

In one segment, Galvez held up a bottle of water that she said came from Mexico City and which she said laboratory tests showed to be contaminated. "I invite you to drink this water," she said, addressing Sheinbaum, who was the mayor of the city until last June.

Sheinbaum, for her part, promised to increase the mininum wage and vowed to improve water recycling in Mexico which has been facing drought and widespread water shortages. She also said she would increase the use of solar panels on domestic homes and electric vehicles for public transport.

She frequently made reference to what she called the achievements of the current government of President Andres Manuel Lopez Obrador, whose consistent popularity has helped give Sheinbaum a strong lead in the polls.

"Mexico is great and will continue to be so with fraternity, more justice, and more prosperity with sustainable development," Sheinbaum said. REUTERS