As Paul Haenle, director of the Beijing-based Carnegie-Tsinghua Center for Global Policy, observes, U.S. presidential campaigns since the late 1970s have evinced an unmistakable pattern: major party presidential candidates will call for a tougher stance on China, which they will promptly abandon on taking office.

Bill Clinton, for example, sharply criticized President George H.W. Bush for his China policy during the 1992 presidential campaign before pivoting to a far more accommodating posture:

Clinton was especially vocal in his pledge to link the granting of the most-favored-nation trade status to China improving its human rights record, criticizing Bush’s decision to unconditionally renew China’s most-favored-nation status.
Once in office, however, Clinton backtracked from this pledge. During his first year as president, he blocked the creation of a bill that would have required China to change its policies on human rights and trade over a one-year period in order to win most-favored-nation renewal the following year.

The next passage is my favorite, as it serves as a reminder of Clinton’s political prowess:

Clinton convinced the bill’s key proponents on Capitol Hill—Senator George Mitchell and Representative Nancy Pelosi—to instead work with his administration to create a presidential executive order that he said would essentially serve the same function. Clinton signed the executive order in the spring of 1993.
However, one year later, Clinton dropped the executive order and stopped linking China’s most-favored-nation trade status with its human rights record, saying it was time to “take a new path” with China.

In a somewhat similar vein, Barack Obama often railed against Chinese economic practices in 2008, e.g., he often referred to China as a currency manipulator.

Almost immediately after taking office, the Obama administration backtracked on its currency manipulator stance. During the new administration’s first days, then treasury secretary-designate, Timothy Geithner, followed campaign-trail talking points and replied to questions raised during his Senate confirmation hearings by stating that China was in fact a currency manipulator. But when pressed by the media as to whether this was official White House position, the administration moved to distance itself from Geithner’s response.
And after nearly four years in office, Obama has yet to follow through on officially designating China a currency manipulator.

And now Mitt Romney is following this same script. Haenle offers thoughts on how a Romney administration might actually proceed, which I recommend reading.