Soldiers in Bogotá, Colombia’s capital on Saturday. President Bush was expected to arrive for a brief visit during his Latin American trip.

By Simon Romero

BOGOTÁ, Colombia — The Bush administration has no closer ally in South America than Colombia, the recipient of more than $4 billion in American aid this decade to combat drug trafficking and guerrilla insurgencies. But a widening scandal tying paramilitary death squads and drug traffickers to close supporters of President Álvaro Uribe is clouding President Bush’s brief visit here on Sunday.

Since the scandal worsened in recent weeks, Democrats in the United States Congress have increased their scrutiny of two important measures before them: a broad trade agreement with Colombia that has already been signed by Mr. Bush and Mr. Uribe, and a request from the administration for a new $3.9 billion aid package for the country.

Claims of human rights abuses by political allies of Mr. Uribe, including the use of information from the executive branch’s intelligence service to assassinate union organizers and university professors, have already resulted in the arrest of Jorge Noguera, a former chief of Colombia’s secret police who was awarded that job after working on the president’s campaign.

“Uribe has certainly been considered a bright light here in the United States, but at some point you have to ask: what are these people doing?” Senator Patrick J. Leahy, the Vermont Democrat who heads the Senate panel that oversees aid to Colombia, said in a telephone interview from Washington. “It’s time to take a pause and look at what we’ve done,” he said, referring to the effectiveness of aid to Colombia.

Senior government officials here say concern over the scandal should not distract legislators in the United States from strides by Mr. Uribe since his presidency began in 2002. Mr. Uribe, an Oxford-educated lawyer, remains highly popular, with a 72 percent approval rating. Many Colombians, particularly in cities like Bogotá and Medellín, have welcomed a break with the chaotic years early in the decade when violence by guerrillas and paramilitaries was more widespread.

“This country was going to be Sudan, and we’ve turned a corner in a dramatic way,” Vice President Francisco Santos said in an interview, referring to fears at one point that Colombia, destabilized by an internal war, could become a failed state.

He pointed to accomplishments like economic growth expected to surpass 6 percent this year, a reduction in violent crime rates in large cities, and a process demobilizing about 30,000 paramilitary combatants.

Mr. Santos expressed gratitude for American help with efforts to end Colombia’s internal war, which has dragged on for more than four decades, displacing three million people.

Still, Mr. Santos turned on its head a statement by Winston Churchill about Americans always doing the right thing after exhausting all the alternatives by saying the United States had made “all the right decisions” in relation to Colombia. “If the Congress doesn’t approve the free trade agreement, the message is that being a friend of the United States doesn’t pay,” Mr. Santos said.

Supporters of Mr. Uribe say ties between paramilitary death squads and political supporters of the president are coming to light because of the resilience of Colombia’s political institutions, particularly the Supreme Court, which has been investigating the connections.

The court’s diligence despite death threats to its members has resulted in startling actions like an arrest warrant issued this month for Álvaro Araújo Noguera, a regional political boss implicated in the kidnapping of a member of a rival political family. Mr. Araújo, the father of Mr. Uribe’s former foreign minister, María Consuelo Araújo, remains at large.

“It is not our concern,” said Alfredo Gómez Quintero, the magistrate at the Supreme Court leading the investigation, when asked in an interview how the revelations might affect American aid to Colombia. “We know the eyes of the world are upon us. Our only job is to arrive at the truth.”

Beyond the paramilitary scandal ensnaring members of Mr. Uribe’s government and at least eight members of his coalition in Congress, human rights organizations are calling attention to the killings of trade union officials in the past six years. And there are claims of abuses involving American companies like the Drummond Company, a coal producer based in Birmingham, Ala.

A judge in Alabama this week allowed a civil lawsuit against Drummond to go forward in which the company is accused of allowing paramilitary gunmen to kill three union leaders at its operations in northern Colombia.

Drummond has repeatedly denied having a role in the killings, which have nonetheless generated skepticism over tightening trade relations with Colombia without safeguarding the rights of the working poor.

“Our aid should be more focused on giving Colombian prosecutors the resources to do their job,” said Representative Jim McGovern, a Massachusetts Democrat who visited Colombia this month.

Here in Bogotá, officials point to Mr. Bush’s visit, the first by an American president to the capital since Ronald Reagan in 1982, as evidence that the security situation has improved. Certainly the scrubbed prosperity of parts of Bogotá, its hotels bulging with foreign business executives and even the occasional tourist, contrasts with the swaths of territory still controlled by leftist-inspired guerrilla organizations.

But Mr. Bush’s visit has also drawn attention to the fact that Colombia, despite being the largest recipient of American aid outside the Middle East and Afghanistan, remains the world’s largest producer of cocaine.

The recent emergence of shadowy new paramilitary organizations with an intense focus on the cocaine trade illustrates the hydra-headed nature of Colombia’s traffickers, political analysts here say.

Mr. Santos, the vice president, said the supply of Colombian cocaine to the United States would be even greater without American antinarcotics aid. Mr. Bush is expected to stand by Mr. Uribe at a time when explicit allies in the region remain scarce.

The fragile stability in Colombia’s largest cities and the slow-burning war in its countryside came into focus in the days before Mr. Bush’s arrival, after Gen. Jorge Daniel Castro, the country’s police commander, said officials had monitored communications by guerrillas about plans for sabotage and attacks to coincide with the visit.

More than 7,000 police officers have been assigned to protect Mr. Bush.