The Colombian government, the recipient of billions of dollars in U.S. aid to fight drugs and a leftist insurgency, is under siege as evidence mounts of links between rightist death squads and dozens of officials loyal to President Álvaro Uribe.

In the past week, the country's Supreme Court summoned six legislators to answer accusations that they had conspired with paramilitary leaders who are alleged to have killed tens of thousands of leftist sympathizers and ordinary civilians and to have run drug trafficking networks since the 1980s.

They are among two dozen sitting and former lawmakers, governors and other public servants being investigated for or charged with colluding with paramilitary death squads to fix elections, plan massacres, share in corruption proceeds or help the militias get a better deal in peace talks.

The so-called parapolitical crisis threatens to close in on Uribe, the best friend President George W. Bush has in a region increasingly dominated by leftist politicians. It also risks setting back Colombian efforts to make peace with armed insurgents on the left and right who have terrorized civilians and trafficked drugs for decades.

Despite the demobilization over the past three years of 31,000 members and allies of rightist death squads, there are widespread reports that their political influence and hold over organized crime and drug trafficking remain intact.

A congressional committee is studying accusations that Uribe himself supported the rise of rightist militias when he was a governor in the 1990s. Uribe has vehemently denied the allegations, challenging anyone with evidence to come forward.

Still, the accusations against Uribe and his allies have reopened old wounds in Colombian society.

Civilian militias formed in the 1980s to combat leftist guerrillas, and they later morphed into death squads that engaged in drug trafficking and extortion. It has long been alleged that powerful elites — from cattle ranchers and politicians to military commanders — helped establish and fund the militias. Several years ago, paramilitary leaders boldly declared that they controlled one-third of the Colombian Congress.

But after years of impunity, the paramilitaries have come under the microscope after disarming and agreeing to confess their crimes in exchange for lenient sentences.

The attorney general's office announced in October that a confiscated computer belonging to a paramilitary leader known as Jorge 40 contained evidence that politicians had accepted funds from paramilitaries, used their links to militias to intimidate constituents into supporting them and even plotted massacres. Since then, fresh revelations, arrest warrants and resignations have followed.

"They are just turning over the first rock to see what worms are under it, and there are many more rocks to go," said Adam Isacson, director of the Colombia program at the Center for International Policy, a think tank in Washington.

"We still haven't gotten to the generals and colonels, the industrialists and landowners or senior members of Congress," he said. "Nobody has any idea how high this will go."

Two weeks ago, a pro-Uribe senator, Miguel de la Espriella, revealed that he and 39 other congressmen had signed a secret accord pledging loyalty to the militias at a meeting in 2001.

With the government's credibility at stake, Uribe is scrambling to salvage his reputation by taking a hard line against the paramilitaries and those who aided them.

On Dec. 1, his government moved 59 top paramilitary chiefs who had been confined at a converted resort to a maximum-security prison, citing rumors that they were plotting to flee and were involved in the murders of two paramilitary commanders who were not in custody. The militia chiefs angrily denied the rumors, and embarrassing allegations surfaced last week that corrupt police and prosecutors might have been involved in the murders.

If charges against the security forces are proved in court, Isacson said, "It'll be really hard for Washington to justify continuing $600 million a year in military and police aid to Colombia."

Uribe's three-year peace process with paramilitaries, criticized by victims' groups for being too lenient, was the centerpiece of his first term.

Coupled with his crackdown on leftist guerrillas and improvements in security, it won him a landslide re-election last May and continued U.S. support. But the confidence between the government and the paramilitaries that allowed for a peace accord appears to have crumbled. Last week, the paramilitary chiefs declared an end to talks with Uribe's envoys.

Security analysts worry that the rupture of trust in the peace process could be taken as a signal by the few thousand paramilitaries who have not demobilized to unleash a new cycle of violence.

Those groups that have not demobilized yet probably will not do so now, said Alfredo Rangel Suárez, director of the Security and Democracy Foundation in Bogotá. "This crisis could set off vendettas and violence among paramilitaries and will implicate more sectors," he said.