While the TPP undoubtedly will boost economic growth in the US, and elsewhere, a little bit, its real significance is geopolitical. It is primarily an effort to counter future Chinese aggression in the South China Sea by creating a rules-based economic alliance to supplement American military power in the region. Failure of the TPP will send a message to countries bordering the South China Sea that America cannot be trusted, and that they have no choice but to make the best deals they can by themselves with their dangerous and unpredictable neighbor. Chinese divide and conquer tactics will consequently prevail, and the military and political environment will become much more unstable.
Hillary undoubtedly knows this, which makes her pandering on the issue all the more depressing. From a tactical perspective, it makes no sense for her to engage in a protectionist race to the bottom with Trump, because, given her history, she cannot win it. She is going to look stupid and weak on this issue during the debates unless she vigorously defends both the concept of free trade and the agreements that she and Bill actually supported. The best case scenario, therefore, is that Hillary limits her objections to essentially cosmetic issues in the TPP during the campaign, wins the election, and fixes the “problems” with the agreement early in her administration.