The United States and Colombia signed an agreement Wednesday meant to knock down trade barriers for U.S. businesses and bolster a South American ally that has been battling drug traffickers and guerrillas.

With trade-wary Democrats soon to control Congress, however, prospects appear dim for ratification — and for other trade liberalization deals as well.

President Bush is seeking congressional approval for trade pacts with Colombia, Peru and Vietnam, and U.S. officials are negotiating agreements with South Korea, Malaysia and others. These efforts are crucial, Bush argues, if U.S. companies are to compete with their foreign counterparts.

Democrats, who will come to power when a new Congress begins Jan. 4, contend the deals often favor foreign competitors, and they accuse the Bush administration of doing too little to protect Americans from unfair foreign trade practices.

In a letter to the administration, several congressional Democrats said the Colombia deal does not protect workers' rights.

Bush rejected Democratic objections during a recent trip to Asia.

"We hear voices calling for us to retreat from the world and close our doors to these opportunities," Bush said in a speech in Singapore, which has a free trade agreement. "These are the old temptations of isolationism and protectionism, and America must reject them."

The U.S. trade representative's office said the deal with Colombia, if approved, would make more than 80 percent of U.S. consumer and industrial exports to Colombia free of duties and improve two-way trade that reached $14.3 billion last year.

When Colombian President Alvaro Uribe visited Congress this month looking to secure backing for the agreement, Democrats bluntly told him its prospects were sinking.

The deal would be the United States' biggest in the Western Hemisphere since the North American Free Trade Agreement in 1994. Democrats point to claims by the International Labor Organization that Colombia is the world's deadliest country for labor organizers, with more than 1,200 cases of murder in recent years.

Jeffrey Schott, a senior fellow at the Peterson Institute for International Economics in Washington, said success for Bush's trade agenda hinges largely on the ability of international negotiators to rekindle stalled World Trade Organization talks aimed at settling a global trade liberalization deal.

"If that occurs and there's the promise of a big, multilateral trade deal, then the president will have a strong rationale of going to the Congress and asking for a renewal of trade promotion authority. Without that, there will be no major trade agreements following up on those that have already been signed," Schott said.

The soon-to-expire trade promotion authority, or "fast track" trade legislation, allows Congress to ratify or reject, but not modify, trade deals negotiated by the White House. Democrats are unlikely to renew the legislation, and that would make it much more difficult for any treaty to gain congressional approval in the United States, the world's largest trading nation.