PITTSBURGH (Reuters) - Democratic presidential candidate Barack Obama vowed on Monday to get tough with China over its undervalued currency, saying he would threaten to limit access to the U.S. market as a bargaining tool with the Chinese.

“What we need to do is just be better bargainers and say ‘Look, here’s the bottom line: You guys keep on manipulating your currency, we are going to start shutting off access to some of our markets,” he told a large crowd of unionized steelworkers in the industrial city of Pittsburgh.

“If you are doing the right thing and not trying to manipulate your currencies to our disadvantage then you will have access,” Obama said to raucous applause from the United Steelworkers and other industrial workers from Pennsylvania.

U.S. President George W. Bush, whose administration has opposed attempts by the U.S. Congress to pass legislation to force China to revalue its currency, was a tough-talking “patsy” on trade negotiations, said the Illinois senator.

“America and the world can benefit from trade with China. But trade with China will only be good for you if China itself plays by the rules and acts as a positive force for balanced world growth,” he said.

Obama was speaking at a forum hosted by the Alliance for American Manufacturing, which has launched a statewide “China Cheats. Pennsylvania Loses” campaign for the April 22 Democratic primary election. Democratic rival Hillary Clinton will also attend the rally on Monday.

The AAM said that Pennsylvania has lost more than 207,400 manufacturing jobs since 2000 -- almost one quarter of roughly all Pennsylvania factory jobs. It cited a study as saying the mushrooming U.S. trade deficit with China alone cost Pennsylvania more than 78,000 jobs between 2001-2006.