Prime Minister John Key says he thinks there's a 50-50 chance that the United States will ratify the Trans-Pacific Partnership free trade agreement if Hillary Clinton wins the presidency.
But if her opponent Donald Trump wins, then there's no chance.
"I think if Donald Trump wins, it's got no chance," he told reporters on Monday.
"But if Hillary Clinton wins, I think there's a possibility, a window, I think it's a bit 50-50 myself."
President Barack Obama, a TPP supporter, hopes the agreement can be ratified by the US Congress during the so-called lame duck period between the presidential election on November 8 and the inauguration of the new president on January 20.
Both candidates have said they oppose the TPP, but Mr Trump is much more strongly opposed to it than Mrs Clinton.
"My sense of it is that in Hillary Clinton's case she has a more nuanced position than Donald Trump," Mr Key said.
"I think she's saying she thought the TPP was gold standard, and now she thinks it's not quite as good a deal - that would argue that she would want to come back to the table."
The 12-nation agreement covers 40 percent of global trade and 800 million people.
To take effect it must be ratified by at least six countries that account for 85 percent of the group's economic output, which makes the US essential.
The partner countries are New Zealand, Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, Peru, Singapore, the United States and Vietnam.