The PM has warned the US it will lose any influence in the Asia Pacific region if it doesn't stick with the Trans Pacific Trade Partnership.

OPINION: John Key's audience at the Council on Foreign Relations in New York was a small but well-heeled one. So a shocked gasp went around the room when one of its armchair generals rose from his seat to ask whether New Zealand was good for much more in the US than a few lamb chops.

His gripe was New Zealand's anti-nuclear legislation and not pulling our weight which, as Key pointed out, is ancient history, given our decade-plus involvement in Afghanistan and now Iraq.

That was not the only history lesson Key felt compelled to deliver to his US audience.

RASHID UMAR ABBASI/REUTERS John Key speaking at the Council on Foreign Relations in Manhattan.

He harked back to the days of "fortress New Zealand", the time pre-1980s when we ran one of the more protectionist economies in the world, our manufacturers and even wine makers propped up by tariffs and quotas that freed them from having to compete with companies overseas who did things faster, cheaper and better.

It wasn't a huge leap to see the parallels between that and where Key saw the likes of Republican presidential candidate Donald Trump and former Democratic contender Bernie Sanders leading the United States with their anti-free trade, anti-China rhetoric.

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MIKE SCOTT/ FAIRFAX NZ Not pulling our weight? Of course we are, Key- pictured here with Kiwi troops in Iraq - tells US audience.

Key's warning to the US is that shutting its doors to the world could have the opposite effect to what Trump promises with his clarion call to make America great again.

As the rest of the world retreats into protectionism, Key is almost a lone voice advocating for free trade and, in particular, the Trans Pacific Partnership deal, on the international stage.

US President Barack Obama is now officially into the "lame duck" period of his presidency and if he can't get the TPP through the US Congress before the election, Key worries there may never be another opportunity.

MIKE SEGAR Would a Trump presidency kill the TPP?

Even Democratic contender Hillary Clinton is talking about renegotiating the TPP - a move that could effectively kill the deal, as it would put everything else up for renegotiation among the 12 member countries.

Key counts New Zealand as among those who would expect new terms if any renegotiation by the US weakens our own position.

Key didn't pull any punches, meanwhile, in warning what would happen if the US pulled out.

Something else would fill the void. And in this case, the something would probably be China, the very bogey used by Trump to scare up opposition to the TPP.

Which is why Key's message to the US was an unusually blunt one - sign the TPP now, or watch America slowly slide from "great", to waning super power.