BOGOTA, COLOMBIA -- Colombia's powerful drug lords, once accused by the Reagan administration of being in league with leftist guerrillas, have been collaborating with extreme right-wing businessmen and some military officers in a wave of assassinations of leftist politicians and dissidents, according to Colombian government and law enforcement officials.

One of the government's top prosecutors, Eduardo Lozano, announced last month that investigators had traced the assassins of leftist union leaders in the Uraba banana region to narco-traficantes, or drug traffickers. Evidence is mounting that these drug lords are behind similar crimes elsewhere.

Some Colombian leaders believe that members of the vast cocaine conglomerate known as the Medellin Cartel are the architects of the deadly conspiracy against the left.

"As property owners, as landowners, as cattlemen, the narcotics traffickers are working with other landowners and other cattlemen and with sectors of the state security forces in unleashing a dirty war against the left," said Sen. Ivan Marulanda, who represents the district in which Medellin is located.

The Medellin Cartel sells cocaine by the ton, controlling 80 percent of the U.S. market and earning as much as $6 billion a year, according to foreign drug experts. They say one-third of those drug dollars are invested in Colombian industry, real estate and agriculture.

Marulanda, a leader of the strongly antidrug reform movement called New Liberalism, said drug traffickers are now Colombia's "largest capitalists and largest landowners."

He said the traffickers see their private property jeopardized by the rising influence of the Patriotic Union, a political party created by the guerrillas of the Soviet-line Colombian Revolutionary Armed Forces, or FARC.

The Patriotic Union has become the most successful leftist party in Colombian history, even though nearly 500 of its activists, including party president Jaime Pardo Leal, have been shot to death in the past two years.

The Colombian government charged yesterday that the mastermind behind Pardo's Oct. 11 slaying was drug czar Jose Gonzalo Rodriquez Gacha. The drug industry was "irrefutably engaged" in the murder, the announcement said. {See accompanying article, Page A22.}

The extermination of Patriotic Union leaders directly threatens President Virgilio Barco's efforts to persuade guerrillas to lay down their arms and enter peaceful politics, but the government appears unable to stop the slayings.

Barco's predecessor, former president Belisario Betancur, recently became the first national leader to declare publicly what government officials say frequently in private. "We are before an organization stronger than the state," Betancur warned reporters earlier this month.

"The river of blood that is running through Colombia . . . appears to be conducted by criminal groups of great economic and logistical power which have the resources to strike wherever and whenever they want," said Betancur, in a veiled reference to the Medellin Cartel.

Colombian judges and police are so paralyzed by the assaults and the bribes of the drug traffickers that Barco is considering a proposal to hire European criminal investigators to crack the nexus between the traffickers and the right wing, according to one Cabinet minister. Meanwhile, the killings continue.

"The murder method is always the same," said a spokesman in the Presidential Palace. "A paid killer who nobody knows flies in from Medellin, commits the killing, and flies out. It's the method developed by the narco-traficantes."

In the past, the Reagan administration charged that the drug lords enjoyed armed protection from leftist guerrillas intent on toppling the Colombian government.

After Colombian police swooped down on a jungle drug laboratory known as "Tranquilandia" in 1984 and seized nearly 14 tons of cocaine, U.S. Ambassador Lewis Tambs made a startling announcement: Marxist guerrillas had been guarding Tranquilandia for the drug traffickers.

He coined a term summing up the unholy alliance -- "narco-guerrillas."

The notion that cocaine kings and communists had joined forces became a principle of Reagan administration foreign policy. A State Department and Department of Defense report in 1985 on Soviet influence in Latin America warned of an "alliance between drug smugglers and arms dealers in support of terrorists and guerrillas."

Although Tambs' view was given credence at the time, the head of the U.S.-financed antinarcotics police, Gen. Miguel Antonio Gomez Padilla, now disagrees that guerrillas were involved in Tranquilandia. Top government officials say that rather than supporting guerrillas, the drug traffickers led by the Medellin cocaine ring are murdering allies of the insurgents, a view shared by western diplomats here.

Notched into the Andes Mountains 150 miles northwest of Bogota, Medellin is Colombia's second-largest city and the headquarters of the cartel.

The cartel became well known in the United States after one of its four leaders, Carlos Lehder Rivas, was captured in February and extradicted to Florida, where he is now on trial.

But Colombians had learned of the cartel's existence six years earlier, in a flamboyant proclamation by the traffickers.

On Dec. 2, 1981, a small plane circled a stadium in the city of Cali during a soccer match and dropped leaflets announcing the formation of "Death to Kidnapers," which became known by its Spanish acronym MAS.

The leaflet said MAS had been founded by 223 leading drug traffickers who were tired of leftist subversives preying on their families. MAS would carry out "the immediate and public execution" of kidnapers, the leaflet vowed.

Three weeks earlier, heavily armed urban guerrillas of the April 19 Movement, or M-19, had entered the University of Antioquia and seized the 26-year-old daughter of Fabio Ochoa Restrepo, patriarch of Colombia's first family of cocaine. The guerrillas demanded $12 million for her return.

MAS counterattacked by kidnaping guerrillas. One M-19 member was chained to a Medellin newspaper building with the letters MAS written across her chest. Another five guerrillas were held until the M-19 released Ochoa's daughter unharmed. No ransom was paid.

Analysts now believe that when the guerrillas attacked the Ochoa family, they jolted the cocaine kings into a new awareness of themselves as a rising social class under siege from the left. MAS was their response.

"MAS represented the {drug} mafia's first organized expression in defense of its economic interests," said Marulanda.

MAS soon shifted its focus from kidnapers in Medellin to political activists throughout the country, letting loose a wave of violence that rolled over Colombia in 1982.

Claiming to be officers of the Army intelligence service B-2, MAS death squads rounded up and shot attorneys, peasant leaders, unionists and university professors.

Evidence of official involvement emerged in January 1983 when three peasants being held on murder charges said they had been recruited into MAS by Army officers and ordered to kill communists.

A month later, Attorney General Carlos Jimenez Gomez stunned the nation with a report charging that 59 active military officers belonged to MAS. Public attention fixed on the highest-ranking officer, Army Lt. Col. Alvaro Hernan Velandia.

During 1982, Velandia, an expert in counterinsurgency, was routing FARC guerrillas from their hideouts in the middle of the long Magdalena River Valley in the country's agricultural heartland.

The region also drew the attention of MAS assassins, who targeted civilians they believed to be supplying the guerrillas. One MAS hit man, Mariano de Jesus Ramirez Garcia, committed 300 crimes, according to police.

Velandia was pursuing FARC supporters in the river town of La Dorada when he arrested a bank employe named Armando Calle, an acknowledged member of the Colombian Communist Party. Velandia accused Calle of feeding financial information to the guerrillas, but a judge released him for lack of evidence.

In an interview with the newsweekly Semana, Calle said that, during subsequent interrogations, Velandia tried to convert him into an informant. When he wouldn't cooperate, the colonel threatened him, Calle claimed.

Four days later, Calle was shot six times by gunmen firing from a motorcycle. He survived, but a bullet entered his spinal cord and paralyzed him for life. He sued Velandia.

Despite the objections of Attorney General Jimenez, the lawsuit and the attorney general's charge that Velandia was involved in MAS were transferred from the civilian judicial system to military courts.

Velandia was declared innocent.

He told reporters that the accusations against him had been fabricated by the leader of Colombia's Communist Party.

Velandia's career has not suffered as a result. He was recently promoted to head the Army Intelligence and Counterintelligence Battalion and has now moved on to another, secret post that Defense Minister Rafael Samudio Molina refuses to identify.

Meanwhile, the FARC guerrillas have moved out of Magdalena Valley. But police sources say drug traffickers have moved in.

Two czars of the Medellin Cartel -- Rodriguez Gacha and Pablo Escobar Gaviria -- have consolidated large haciendas around La Dorada and the nearby town of Puerto Salgar, police say.

The cartel has other reasons to rest easy these days. In June, an independent jurist appointed by the Colombian Supreme Court struck down the extradition treaty that sent Lehder to the United States.

Although Jorge Luis Ochoa Vasquez, Rodriguez Gacha and Escobar -- the three remaining drug kingpins -- are under indictment in the United States, they face no arrest warrants in their own country and are free to further their political interests.

The Medellin Cartel has been heavily involved in politics for years. Lehder launched a local neo-fascist movement, and Rodriquez Gacha campaigned for the Conservatives in 1982 and the Liberals in 1986. But Escobar had the greatest success.

In 1982, he was elected alternate deputy to Parliament for the Liberal Party. He became a financial backer of party leader Alberto Santofimio Botero, who was recently appointed president of the party's national committee.

Escobar was eventually hounded out of office by then-senator Rodrigo Lara Bonilla. Lara was murdered in April 1984, one month after the fall of the Tranquilandia cocaine laboratory. His death so shocked Colombia that the drug traffickers had to retreat from public life.

Guillermo Cano, editor of the Bogota daily newspaper El Espectador, said the drug kings now want "to recover the political space they had in the days when Pablo Escobar was a deputy.

"Their economic power is very great," said Cano. "But they want political power. They want to be the owners of the state. They see the left as an obstacle."