The deal would permanently assure Costa Rica of tariff-free exports to the US, especially textile and tuna.
"If this deal is approved, it won't make us better or worse. Just richer"
Oscar Arias, Costa Rican president
Supporters, led by Oscar Arias, the president, say Costa Rica needs to open its economy more since it is a small country with few natural resources.
Arias, who won the Nobel Peace Prize for helping end Central American civil wars in the 1980s, says Cafta will help Costa Rica stay ahead in the region.
"If this deal is approved, it won't make us better or worse," he said last week. "Just richer."
But opponents, 100,000 of whom turned out for the largest march in the country in years last weekend, say the agreement would be a disaster for local farmers.
They say it will weaken the country's prized welfare system - among the strongest in Latin America – and threaten the supply of cheap medicine and a state-run telephone company.
Population of 4 million
Boasts of pristine beaches and jungles
Has enjoyed almost uninterrupted democratic government for more than a century
Has better education and healthcare than neighbours
Coffee farming, tourism, call centres and microchip manufacturing support growing and diversified economy
Has no army
Attracts migrants from Nicaragua and Panama
It will also let in a flood of cheap US farm imports and limit the country's sovereignty by taking investment disputes to international arbitration, critics charge.
"It seriously hurts the poor and it has benefits for the wealthy class," 29-year-old student Luis Sanchez said.
A poll last week in La Nacion newspaper showed Costa Ricans rejecting the trade deal by 55 per cent to 43 per cent.
Other recent surveys showed the 4 million people in the most prosperous and stable country in Central America sharply divided.
Even if the voters say "yes", Arias still must push more than a dozen implementing laws through his legislature, which has already voted to reject the treaty.
The White House has warned that it will not renegotiate the deal if Costa Ricans vote against the current proposal.