By JUAN FORERO

PIVIJAY, Colombia — As some 23,000 paramilitary fighters have disarmed here over the last two years, their top commanders have declared their intentions to play a role in politics.

But signs are emerging that the role is a dark one, as commanders use bribery and intimidation to control local lawmakers and even blocs of representatives in the Congress while they reshape their militias into criminal networks that traffic in cocaine, extort businesses and loot local governments.

Warnings of these activities have come from sources as varied as Colombian politicians, the United Nations, Western diplomats and human rights groups. Colombia's Supreme Court has begun investigating ties between paramilitaries and Congress, and some parties have begun expelling representatives with links to the groups.

The demobilization of the far-right antiguerrilla paramilitaries — a disarmament far larger than those that ended Central America's wars in the 1990's — is part of a deal the government had hoped would bring peace to areas long tormented by war and drug trafficking. But there is little oversight of the process, and no mechanism to guarantee that the militias are completely dismantled.

"Usually the goal of a peace process is to get armed combatants to lay down arms and enter politics," said Cynthia Arnson, director of the Latin America program at the Woodrow Wilson International Center for Scholars in Washington. "It's another thing to do it through threats and extortion and bribery and the other ways that they've used to influence politics."

Here along the Caribbean coast, the power of the paramilitaries and the methodical tactics they used to take political control were evident in 2002, in the last congressional elections. The victors won in abnormal landslides of up to 95 percent of the vote. Mayors and councilmen were also swept in, particularly in this state, Magdalena, where candidates for local offices ran unopposed in 14 of 30 towns.

Dieb Maloof, 41, who was re-elected to his Senate seat with 84 percent of the vote in this cattle town, denied any manipulation. "People voted for me because we did the political work," he said in an interview.

But the Western diplomats and Colombian lawmakers, warning that the paramilitaries are preparing to strengthen their stranglehold, say Mr. Maloof, a handful of other members of Congress and some mayors met secretly in December with Rodrigo Tovar, a paramilitary warlord, to lay the groundwork for an electoral sweep on March 12. "In those meetings, they discussed what they were going to do, who was going to be a candidate, what strategies they would use, even whom they would threaten," said César Gaviria, a former president who leads the opposition Liberal Party.

And El Tiempo, the largest paper, has reported meetings between paramilitaries and members of Congress in two other states. The members have denied the accusations.

In January, Mr. Maloof was expelled from his party, as were four other members of Congress. The five belonged to two parties closely tied to President Álvaro Uribe.

In February, two other representatives who openly expressed sympathy for paramilitary commanders were removed from a third party.

President Uribe, who remains popular in Colombia for battling the rebels, has hailed the expulsions, saying the government will not "allow illegal funding, the bribing or intimidation of leaders or voters, or the participation of violent groups in the political campaigns."

And Vice President Francisco Santos, while admitting problems, said the demobilization was working. "Success comes at the end," he said, "but I think it is on the right path."

The expelled members of Congress, however, have simply shifted their membership to other conservative movements and are expected to easily win office. By pulling the levers of power behind the scenes, said Gustavo Duncan, a security analyst finishing a book about paramilitary influence in Colombian institutions, the commanders are out to achieve ambitious goals, like repealing an extradition treaty with the United States that threatens them. While the Bush administration has supported Mr. Uribe, its ambassador has warned of the manipulations of politics, and Congress has offered only token sums for the demobilization.

"Some people believe that this process risks legitimizing and even institutionalizing the corrupt and illegal enterprises and the role they are playing in the economic and political life of the country," Tim Rieser, foreign policy aide to Senator Patrick J. Leahy, a Vermont Democrat and ranking member of the subcommittee on foreign operations, said after visiting Colombia in December.

Perhaps no Colombian state has suffered through the violence and political manipulation of the paramilitaries as much as Magdalena. The paramilitaries consolidated power by embarking on a terror campaign in the late 1990's, massacring villagers and assassinating leftist politicians and union leaders.

Then their supporters began to sweep into office virtually unopposed, according to an exhaustive study of voting trends by Claudia López, a political analyst working for Semana magazine's Web site.

Some public officials here, like Pivijay's mayor, Carlos Alfonso Severini, acknowledge that paramilitaries have a strong military influence in the region. But not so in politics, he contended in an interview in his office. "They don't choose the candidates," he said. "We, the political leaders in the towns, do it."

But in whispers, townspeople and independent politicians talk of the top-to-bottom control of Mr. Tovar, the scion of a politically connected, land-owning family who is among the most feared paramilitaries.

Congressional hopefuls like Aroldo Guardiola, 50, a law professor who openly defies the paramilitaries, say Mr. Tovar's hold is so strong that he simply cannot venture into much of Magdalena to campaign. And no one expects that grip to loosen as Mr. Tovar's paramilitary fighters disarm in the coming days.

"These days, we can say that democracy in Magdalena has been hijacked," Mr. Guardiola said. "There are municipalities we visit, but we cannot get out into the countryside. It's a danger because what is at stake is not just your career, but your life."

In his elegant home in Barranquilla, Senator Maloof adamantly denied recent news media reports accusing him of being at a reunion where the plot to murder a prosecutor was discussed. He blamed rivals for planting stories.

"When you're in a country that has had 50 years of conflict, no one — I mean no one — can cast the first stone," Mr. Maloof said, saying he planned to keep running for office.

"If God permits it, and people want it, and we don't have to withstand more slander, then we think we will get an extraordinary vote," he said. "We're so optimistic."