Behind Obama's Tough Stance on China's Weak Yuan Policy

The explanation is buried in paragraph 7 of NYT's coverage of the possible policy shift:

The more aggressive position will be popular with organized labor in the United States, a major supporter of Mr. Obama’s presidential campaign, and with many manufacturers who say China is purposely keeping its currency devalued against the dollar and leaving American exports at competitive disadvantage against lower-priced Chinese goods.

Of course, demanding a weak dollar policy, from the country that has been absorbing billions of US debt,is a bit weak in the logic department.