By Miguel Gutierrez and Dave Graham
MEXICO CITY (Reuters) - Mexico's ruling conservatives have stepped up attacks on presidential front-runner Enrique Pena Nieto in an effort to help their struggling candidate, who is slipping back toward third place.
A new TV spot by the National Action Party, or PAN, describing Pena Nieto as a "liar" began to circulate just as a poll showed support for PAN contender Josefina Vazquez Mota falling to its lowest level since the campaign began.
Pena Nieto, of the opposition Institutional Revolutionary Party (PRI), has a commanding lead over Vazquez Mota, whose bid has been undermined by squabbles within her party and a string of mishaps on the campaign trail ahead of the July 1 election.
The advertisement focuses on Pena Nieto's governorship of the State of Mexico, a populous region flanking much of the capital, and accuses him of failing to complete two public works projects he had promised during his 2005-2011 tenure.
"Pena Nieto is a liar. He doesn't keep his word," a male voice says in the spot reminiscent of ads the PAN unleashed against the longstanding 2006 front-runner Andres Manuel Lopez Obrador, who eventually lost to Felipe Calderon in a tight finish.
However, Pena Nieto has a much bigger lead over Vazquez Mota than Lopez Obrador enjoyed over Calderon at the same stage in 2006, and the PAN candidate is losing, not gaining ground.
The latest daily voter survey by pollster GEA-ISA put support for Vazquez Mota at 18 percent, just 1.5 percentage points ahead of left-wing candidate Lopez Obrador in third. Pena Nieto was way out in front at 40.1 percent.
Pena Nieto dismissed the PAN attack at a rally in the southern city of Oaxaca. "These types of campaigns aim to confuse and are not at all based on the work we did in the State of Mexico," he told reporters.
Federico Berrueto, director general of pollster Gabinete de Comunicacion Estrategica, said the PAN's negative strategy smacked of desperation and could even backfire on Vazquez Mota.
"I'd describe it as a last resort," he said. "Even if it ends up being successful it wouldn't surprise me if the one who profits from it is Andres Manuel Lopez Obrador."
The PAN accusations rang hollow because Mexican voters know that governors cannot always fulfill all their promises - and because the PRI could use the same tactic on Calderon and point to where his PAN administration had failed, Berrueto said.
The TV broadside targets the image the PRI has carefully built up around Pena Nieto of a politician who delivers. His reputation as governor was forged by pledges he signed, then checked off as completed during his term in power.
Roberto Gil, Vazquez Mota's campaign chief, said the PRI should brace itself for more of the same.
"We're going to show that Enrique Pena Nieto's campaign, based on the idea that he honors his pledges, is completely false, because the only thing that's absolutely certain is that Pena Nieto is lying," Gil told Mexican radio.
Pena Nieto supporters at the Oaxaca rally were unfazed by the latest offensive against the PRI hopeful.
"It's understandable you get these things in the fight to win, but I don't think the PAN's dirty campaign will affect Pena Nieto," said Arturo Aquino, a local vendor. "Those of us who are going to vote for the PRI won't change our vote."
The PRI ruled Mexico for 71 years straight until the PAN ousted it in 2000. The PAN's popularity has suffered due to a failure to create enough jobs, and a rise in drug-related violence that has marred Calderon's term in office.
Calderon is barred by law from seeking a second term.
While the PAN slings mud at its rival, the PRI's cause is benefiting from a much slicker public relations machine than in 2006, when the party was divided and finished a distant third.
"Their publicity is very impressive," said Ulises Beltran of polling firm BGC. "It was a disaster six years ago."
(Editing by Eric Beech)