President George W Bush has called on Congress to pass a controversial free trade deal with US ally Colombia to help promote regional stability.
Some Congress members are opposed, citing concerns over workers' rights.
Mr Bush suggested the deal could help counter the influence of Venezuela's Hugo Chavez, with whom both the US and Colombia have difficult relations.
The US Senate passed a similar deal with Peru on Tuesday. Deals with Panama and South Korea are pending.
Mr Bush's comments came after he was asked to react to the result of Venezuela's referendum on Sunday, which saw Mr Chavez's proposed constitutional reforms defeated.
"The Venezuelan people rejected one-man rule. They voted for democracy," Mr Bush said.
Among the main proposals before Venezuelans were moves to allow the president to stand for re-election indefinitely and to end the central bank's autonomy.
Mr Bush said the US could make a difference in terms of Venezuelan influence if Congress were to pass a free trade agreement with Colombia.
Failure to do so could destabilise the region, he said.
"The biggest fear in South America is not the leader in Venezuela, but the biggest fear for stability is if the United States Congress rejects the free trade agreement with Colombia," Mr Bush said.
"It would be an insult to a friend; it would send a contradictory message to a country led by a very strong leader, who is working hard to deal with some very difficult problems," Mr Bush said, referring to Colombian President Alvaro Uribe.
Stalled moves
The US and Colombia signed the free trade accord in November 2006, the biggest in the Western Hemisphere since the 1994 North American Free Trade Agreement (Nafta).
The deal, if passed, would make more than 80% of US consumer and industrial exports to Colombia duty free. Trade between the two was worth $14.3bn (£7bn) in 2005.
However, opponents of the accord argue that Mr Uribe's administration has not done enough to protect the rights of workers and trade unionists.
Plans to create a Free Trade Area of the Americas (FTAA), stretching from Canada to Chile, ran into the buffers before their 2005 deadline could be met.
Since then, there has been more emphasis on securing individual trade deals. But these, too, have proved tricky.
In May, Congress and the White House reached agreement on a series of standards for countries entering free trade deals with the US, in a bid to smooth ratification.
In a boost to Mr Bush's trade policy, on Tuesday the Senate passed a deal with Peru - the first to be approved since the Democrats took control of Congress, and the first under the new standards.
The agreement confirms Peru's duty-free access to the US market, while phasing out Peruvian tariffs on US agricultural and manufactured goods.
The White House is also hoping that Congress will approve agreements with Panama and South Korea next year.
Whichever deals are approved, free trade is set to be a significant issue in the 2008 US presidential election, amid a general debate over whether it is in fact in the country's interest.
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