Prime Minister John Key says he's not worried about the fate of the Trans-Pacific Partnership (TPP) agreement, despite the front-running US presidential candidates expressing doubts over the deal.

Mr Key says he believes Hillary Clinton will most likely win the Democratic nomination to run for president, but it's harder to pick who she will be up against.

"It looks like there's momentum building on both sides, it's probably a little clearer on the Democratic side that eventually Hillary will get through," he told the Paul Henry programme this morning.

"The thing with Trump is, I don't think I would have picked him getting as much momentum as he has, but Super Tuesday - anything can happen if he comes through Super Tuesday with really big numbers. It becomes really difficult for them to turn it around, but having said all that it depends on a lot of different factors and I don't think it's impossible that [Marco] Rubio picks up a fair bit of support."

Both Ms Clinton and Mr Trump have expressed doubts about the TPP. Last year Clinton said the deal "didn't meet [her] standards", and Mr Trump has called it a "horrible deal".

"Trump is playing to a largely blue collar vote who feel threatened by manufacturing in China. The funny thing is, actually manufacturing's coming out of China back into the United States, just like it's coming back into New Zealand," says Mr Key.

Despite the opposition, Mr Key says he is confident the deal will get signed off in the US.

"I think Obama can get it through, and secondly, to be blunt, on the campaign trail you see these people say a hell of a lot of stuff and when they get in to office it's a very different deal, both positively and negatively, because in the end it's about the Congress and the Senate and those opposed to the deal."

Opponents have criticised the secretive nature of the deal's negotiations. There have also been concerns the agreement would negatively affect the New Zealand Government's drug-buying agency Pharmac, and open up the country to lawsuits from foreign corporations.

Mr Key says the deal could boost New Zealand's economy by $2.7 billion a year by 2030.

"As I've said about the TPP so many times, it's a good thing, it's not just good for New Zealand, it's actually good for the United States of America. The whole reason Obama's doing it is to give them a pivot into Asia, in other words an opportunity in Asia."

As well as New Zealand and the US, the deal has been signed by Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, Peru, Singapore and Vietnam.

The agreement must now be ratified by the countries involved if it is going to come into effect.

Newshub.