Rachel Rothschild writes in Business Insider that the Trans-Pacific Partnership (TPP) could become a big problem for Hillary Clinton’s campaign.

Hillary Clinton and many of her fellow Democrats meeting in Philadelphia hope to show the party unity arguably lacking when the Republicans gathered in Cleveland.

A sticking point to a unified Democratic Party, however, has been the Trans-Pacific Partnership (TPP), one of the most contentious topics of debate between supporters of Clinton and erstwhile rival Bernie Sanders.

Despite significant lobbying, Sanders failed to secure opposition to the TPP in a draft of the Democratic Party platform.

As a result, the TPP could become a key issue in the election this fall. Donald Trump’s antipathy to the deal is well-known, and he managed to convince “the party of free trade” to drop any mention of the TPP in its party platform, whether in support or opposition. And exit polls from the primaries showed that antitrade sentiment was a key driver of support for Sanders over Clinton.

But, as my research on the history of environmental science and diplomacy highlights, there’s one significant drawback of the TPP that neither side has talked about: how it departs from a half-century of diplomatic progress on the environment and human rights.

The TPP, a trade accord among a dozen Asia-Pacific countries, has received significant criticism from both sides of the political spectrum. The main concern has been the potential impact on American workers and the U.S. economy.

However, the TPP has other glaring deficiencies that also deserve attention by Congress and the presidential candidates: the poor protection given to the environment, food safety and human rights. There is not a single mention of climate change or human rights in the treaty text. Conservation protections for wildlife are minimal at best.

Foreign companies are granted the ability, for example, to challenge environmental laws of other countries through a controversial clause known as the investor-state dispute settlement system. And the TPP’s environment chapter is weaker than those in previous free trade agreements.

Because of these flaws, more than 1,500 organizations, including major U.S. environmental and humanitarian groups, have come out against the TPP.

While leaked documents from the TPP negotiations suggest the U.S. acquiesced to other countries attempts to weaken the agreement, history shows that tying environmental and human rights issues into larger strategic agreements actually strengthens these treaties.

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