A crucial vote affecting the Trans-Pacific Partnership (TPP) will be held in the United States House of Representatives overnight.
The House will decide whether to give President Barack Obama's Administration "fast track authority".
Officially known as Trade Promotion Authority (TPA), the trade deal among 12 countries including New Zealand could be presented back to the House in the future for a yes or no vote, without amendment.
The likely effect would be to give US Trade Representative Mike Froman greater clout in the concluding rounds of negotiations.
The vote is scheduled to be held between 3am and 4am on Saturday (NZT).
It has a companion bill, a Trade Adjustment Assistance bill (TAA), which would give assistance to workers and industries displaced by the effects of free trade agreements.
Essentially TPA is supported by Republicans and opposed by most Democrats. The TAA is supported by more Democrats than Republicans.
Executive director of the New Zealand International Business Forum Stephen Jacobi said if TPA was not passed tomorrow morning, he believed the Republican leadership would try to have another go at getting it passed before the summer recess in August.
That would still give time for legislation to be drafted and passed before the end of the year.
"I think most people would think if you couldn't get the whole thing wrapped up by the end of the year, it's a goner for the time being and would be kicked down the road for another president."
Negotiations for the TPP began in March 2010.
The deal covers more than just market access and tariffs; it includes intellectual property rights, foreign investment rules, labour and environment standards, procurement policies, state-owned enterprises and competition, and disputes procedures.
The countries involved are the US, Canada, Mexico, Peru, Chile, Australia, New Zealand, Singapore, Malaysia, Brunei, Vietnam and Japan.