WASHINGTON (Reuters) - The United Steelworkersunion said Wednesday it planned to file a comprehensivetrade case against "protectionist and predatory practices" usedby China to support its clean energy sector.

The case, which the union plans to detail Thursday,could ratchet up trade tensions across the Pacific at a timewhen many U.S. lawmakers are frustrated with China's currencyand trade policies.

"China has utilized hundreds of billions of dollars ofsubsidies, performance requirements, preferential practices andother trade-illegal activities to advance their domination ofthe (alternative and renewable energy) sector," thesteelworkers said in statement announcing their plans for anews conference to discuss the case.

The union is filing a nearly 6,000-page petition with theU.S. Trade Representative office asking it to launch aninvestigation into the group's charges.

It outlines "five major areas of protectionist andpredatory practices utilized by the Chinese to develop theirgreen sector at the expense of production and job creation herein the U.S.," the steelworkers said.

By law, the Obama administration has 45 days from the dateof filing to determine whether to accept the petition forfurther action, the steelworkers said.

That would make Oct. 24 the latest date for a decision, or10 days before the Nov. 2 congressional elections in whichPresident Barack Obama's fellow Democrats are battling to keepcontrol of both chambers of Congress.

The steelworkers' union was the driving force behindObama's decision last year to slap duties on tires from Chinato stop a surge they said they threatened U.S. job losses.

They have also joined with U.S. steel, paper and aluminummanufacturers to bring other trade cases against China.

Obama has identified the clean energy sector as an enginefor future U.S. economic growth, and has hoped to tap into thehuge market in China to help fuel that.

U.S. Commerce Secretary Gary Locke in May led nearly twodozen U.S. companies on a clean energy trade mission to China. (Reporting by Doug Palmer; Editing by Peter Cooney)