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The partnership includes the US, Japan, Vietnam, Malaysia, Singapore, Brunei, Australia, New Zealand, Canada, Mexico, Peru, and Chile. All together, these countries represent 40% of the world's GDP.
Proponents argue that the deal includes provisions that will raise regional standards on labor, environmental regulations, and intellectual property.
The deal has major geopolitical implications for the US. China, which is conspicuously absent from the list of signatories, would likely see its influence in the region diminished, allowing the US to assert itself as a trade leader.
President Obama has made TPP a priority for his final months in office, and is hoping he can push it through Congress with the help of pro-trade Republicans like John Kasich and Mitch McConnell. But the deal has plenty of detractors.
Hillary Clinton and Donald Trump both say they oppose TPP. It's one of the few issues on which they are in agreement. Notably, both oppose the deal at a time when protectionism has grown quite popular stateside as American workers continue to worry about losing jobs to other countries.
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Where Clinton stands on TPP
Clinton has publicly opposed TPP since October 2015, when the text of the deal was finalized.
"I oppose it now, I'll oppose it after the election, and I'll oppose it as president," she said at acampaign rally in Michigan in August.
But her opposition marked a departure from the praise she gave the deal during her tenure as secretary of state. She once said TPP "sets the gold standard of trade agreements."
"I did say, when I was secretary of state, three years ago, that I hoped it would be the gold standard," Clinton said during a Democratic primary debate in October 2015. "It was just finally negotiated last week, and in looking at it, it didn't meet my standards. My standards for more new, good jobs for Americans, for raising wages for Americans."
"And I want to make sure that I can look into the eyes of any middle-class American and say, 'This will help raise your wages.' And I concluded I could not."
Clinton has also expressed concern over the deal's failure to crack down on currency manipulation, which she claims "kills American jobs," and its lack of provisions to extend patent protection to drug companies in poor countries, which she said would put "the interests of drug companies ahead of patients and consumers."