By Jason Lange

MEXICO CITY, May 7 (Reuters) - A heated debate over the future of Mexico's oil industry will drag on for months, delaying President Felipe Calderon's plans to give foreign companies a bigger say in the country's sagging energy sector.

Calderon aims to patch up faltering output in the world's No. 6 oil exporter by allowing more private participation in the cherished oil sector, nationalized 70 years ago.

It is the biggest and riskiest reform attempt of Calderon's 18-month-old presidency and he had hoped to have it approved in Congress in April.

Instead, leftists have forced him into holding a long "national debate" in the Senate until at least late July, featuring not only politicians but also academics, industry experts and other public figures.

Energy Minister Georgina Kessel kicks off the debate on Thursday, presenting the government's case that Mexico needs to allow private firms to join with state oil monopoly Pemex to seek oil in deep waters of the Gulf of Mexico.

The leftist opposition fears that Calderon's plan will mean a creeping privatization of Pemex and admits that its strategy is to drag out the debate as long as possible. A vote might not happen until August or later.

"(We are) slowing things down," said Sen. Pablo Gomez of the leftist Party of the Democratic Revolution.

Mexico is a top supplier of crude to the United States but output has fallen in recent years as its offshore Cantarell field declines.