The Prime Minister is pinning his hopes of getting the Trans-Pacific Partnership (TPP) across the line on Hillary Clinton and the #NeverTrump wing of the Republican Party.
The controversial trade agreement was signed in Auckland earlier this year amid rowdy protests, but won't come into force until each of the 12 member countries has ratified it.
It's hit a fairly significant hurdle in the US, however - both major presidential candidates, Ms Clinton and Donald Trump, have said they're against the TPP.
Mr Key doesn't think it's dead, but admits it's on life support, awaiting the outcome of the November 8 presidential election.
"If Trump wins, it won't get through. If Clinton wins narrowly, it'll be quite difficult," he told Paul Henry on Monday.
The "easiest" path to ratification would be an emphatic win for Ms Clinton, in his view. She once called it the "gold standard" in trade agreements, only turning against it while fighting off a strong challenge from Bernie Sanders for the Democratic nomination.
Her predecessor Barack Obama has been a big supporter of the TPP.
There is a chance she could arrive in office to find it already ratified, which is Mr Key's plan B. The US House of Representatives is controlled by the Republicans, whom Mr Key sees as being more open to international trade - despite Mr Trump's threat to rip up existing agreements, including the TPP.
The TPP could get through the US Congress during the lame-duck session between Ms Clinton's election and her taking of the reins in January, under the leadership of Speaker Paul Ryan, a Republican.
"It's all about Paul Ryan, who has got a great possibility of one day being the President some point down the track, he has been very pro-free trade," says Mr Key.
"A lot of the arguments that got put to me while I was in the States were that yes it's difficult, but Paul Ryan, he'll be the deciding factor."
Mr Ryan and Mr Trump have a fractious relationship, with the wonkish Mr Ryan taking almost a month to endorse the outspoken real estate billionaire after he wrapped up the Republican nomination for President.