America's top trade official has admitted the Trans-Pacific Partnership mega trade deal with Australia and other Pacific rim nations would be in jeopardy if the US Congress does not pass it during President Barack Obama's final months in office.

Presidential candidates Hillary Clinton and Donald Trump oppose the 12-nation TPP and support from Republicans, who control Congress, is waning during what has been a volatile election year.

"If it's not done this year, it's quite unclear when it would get done, given the broader political developments," US Trade Ambassador Michael Froman said on Thursday.

Republican Senate Majority Leader Mitch McConnell said this week the prospects of a TPP vote this year were "pretty slim".

Mr Obama, pro-TPP legislators and business groups, including the US National Cattlemen's Beef Association, The National Association of Wheat Growers and the US Apple Association, are in a race against time to get the deal passed.

It would be the largest regional trade deal in history, with the US, Australia, New Zealand, Canada, Japan, Mexico, Brunei, Chile, Malaysia, Peru, Singapore and Vietnam signatories.

Mr Froman, who was speaking at a breakfast in Washington DC organised by the Christian Science Monitor, remained confident the TPP could be passed this year.

"We think there is a window of opportunity to get it done," Mr Froman said.

The US presidential election is on November 8.

Mr Obama leaves the White House on January 20 next year and approval of the TPP would be considered one of his great achievements during his eight years in office.

The stakes are high for the US, with a recent Peterson Institute of International Economics study estimating failure to approve the TPP would cost the US about $US94 billion in its first year.

The TPP is also viewed as strengthening America's position in the Asia-Pacific while decreasing China's influence.

"At the end of the day, I don't think Congress is going to want to be responsible for handing the keys of the castle over to China," Mr Froman said.

Australian Trade Minister Steven Ciobo, in the US for meetings with Mr Froman, told the ABC he remains "cautiously optimistic" congress would pass the TPP.