Jan. 28 (Bloomberg) -- A Mexican opposition party threatened to boycott congressional sessions in a bid to hinder an energy reform package backed by President Felipe Calderon.

Congressman Alejandro Camacho of the Party of the Democratic Revolution told reporters today that lawmakers from his party may walk out or resign over a plan to ``sell Mexico'' by opening up its oil industry to private or foreign investment.

Calderon aims to open the state-controlled oil industry to investment in order to free up cash at Petroleos Mexicanos, the state oil monopoly, so it can invest in exploration. Former presidential candidate Andres Manuel Lopez Obrador, a member of the Party of the Democratic Revolution, has called on supporters to demonstrate against the proposal.

``This is going to be a historical battle,'' said Layda Sansores, a lawmaker from the smaller minority party Convergence, which also opposes Calderon's plan.

The president's party will introduce a bill next month to ease the 69-year-old policy of state control over the industry.

Daily crude output could drop to 2.1 million barrels by 2016 from about 3.1 million barrels now unless Pemex gets the money it needs to explore in waters deeper than 5,000 feet in the Gulf of Mexico, where much of Mexico's oil is located, according to the government.

To contact the reporter on this story: Adriana Lopez Caraveo in Mexico City at adrianalopez@bloomberg.net; Jens Erik Gould in Los Angeles at jgould9@bloomberg.net

Last Updated: January 28, 2008 20:22 EST