airport@ (Adds quotes, details on existing contracts, context)
MEXICO CITY, March 22 (Reuters) - The leftist front-runner for the Mexican presidency said on Thursday he would file legal challenges to block future work on the capital's new $13 billion airport, lashing out at the project and its corporate defenders as "corrupt."
Former Mexico City Mayor Andres Manuel Lopez Obrador, who has a double-digit lead in polls for the July 1 election, has previously threatened to scrap the airport, which has been under construction since 2015 and is meant to be finished by 2020.
Lopez Obrador said the airport, the biggest public works project underway in Mexico, was running behind schedule and over budget, and that it could end up being a money pit for his administration if he wins office.
"We will present a legal challenge to stop contracts being awarded, because they are awarding contracts to tie down the next government," Lopez Obrador told reporters in Mexico City.
Earlier this month, the powerful CCE business lobby urged Lopez Obrador to respect the airport contracts or risk damaging investment in Latin America's No. 2 economy.
But he rejected such arguments about the airport, which is being built on the drained bed of Lake Texcoco by the capital.
"I consider the building of the airport to be corruption. And those defending the building of the airport in Lake Texcoco are involved in the business of corruption," Lopez Obrador said.
The company responsible for building the new airport, GACM, did not respond to a request for comment.
The CCE responded with a statement that said it believed the airport was indispensable for the country, and that all legal and transparency requirements should be met.
Lopez Obrador did not give details on how he believes the alleged corruption took place, and he dismissed concerns that halting the contracts could create uncertainty.
"Yes, that's what they say: uncertainty. But then what? To prevent uncertainty I'm going to become an accomplice to corruption? No. I'd prefer there to be uncertainty," he said.
The 64-year-old also said Mexico cannot afford the price tag on the new airport, and that the bed of Lake Texcoco is sinking.
Under his plans, the bonds issued to fund the airport would be guaranteed and existing contracts would not be affected, he says. The current airport should instead be kept and a small one further north of the city should be expanded, he argues.
That plan is not viable, the government says.
Federico Patino, the chief executive of GACM, told an infrastructure conference on Thursday, before Lopez Obrador's comments, that $6 billion had been raised through bonds and that the group was seeking a credit line of $1 billion.
If Lopez Obrador does win the election, it will raise serious questions about the future of the new airport and the significant investments already made in it.
Billions of dollars worth of contracts have been awarded for the airport, which aims to ease the strained capacity of the current hub and improve connectivity.
The largest existing contract by far was awarded to a consortium including a construction company controlled by Mexican tycoon Carlos Slim, one of the world's richest men.
The 85 billion peso ($4.6 billion) contract was for a group including Spain's Acciona, Slim's Operadora Cicsa and several Mexican firms, including builder ICA.
GACM is hoping to raise 30 billion pesos on Friday in an initial offering of an infrastructure investment trust to fund construction of the hub, whose planned terminal building was designed by British architect Norman Foster and Slim's son-in-law.
Runner-up in the last two presidential contests, Lopez Obrador has spent years railing against corruption. Public anger over graft scandals has battered the credibility of the ruling Institutional Revolutionary Party, or PRI. ($1 = 18.5219 Mexican pesos) (Reporting by Adriana Barrera, Daina Beth Solomon, Christine Murray and Sharay Angulo Editing by Dave Graham and Rosalba O'Brien)