The Colombian government of Alvaro Uribe, the president, has also been accused of having ties to drug-dealing and paramilitary groups.
Harry Reid, the Democratic leader of the senate, said Bush was sending the agreement to congress prematurely because Colombia had not addressed concerns about violence against trade unions.
"By sending up the Colombia FTA [free trade agreement] legislation under circumstances that maximise the chances it will fail, he will be adding one more mistake to his legacy and one more mess for the next president to clean up," he said.
Continuing violence
The Colombia pact had also featured in the race for the US Democratic presidential nomination over the weekend, when Hillary Clinton's chief strategist resigned under fire for meeting a Colombian diplomat to discuss the pact, which Clinton opposes.
Some paramilitaries have disarmed but Uribe
continues to be linked to death squads [AP]
Uribe said in Bogota that he hoped for bipartisan support of the pact in congress.
"I want this message to get to the US congress: I beg you to look at the current problems [in Colombia] and the favourable evolution that Colombia has experienced."
Since taking office in 2002, Uribe has demobilised some paramilitaries under an amnesty programme.
But the unions say Uribe's administration has encouraged assassinations of trade unionists who cause problems for companies.
"It tries to stigmatise us, it tries to paint us as rebels, and that's when the right-wing death squads try to kill us," said Fabio Arias, vice- president of Colombia's largest trade union federation.
"These death squads still work with parts of the military and police to kill trade union members in Colombia."