A regional director for Colombian President Alvaro Uribe's re-election campaign resigned on Friday after being signaled by U.S. authorities for links with drug traffickers, the latest casualty in a widening corruption scandal.

Raul Montoya, the president's chief strategist the Caribbean province of Magdalena, is the fourth adviser or member of the Uribe administration to resign this month because of alleged ties to drug traffickers and right-wing paramilitaries.

In a public letter to Uribe's campaign manager, Montoya denied any wrongdoing, stating that no charges had been brought against him either in Colombia or abroad. He said he was quitting his post in order not to affect the president's re-election chances.

Despite seeing his 60 percent to 70 percent popularity shaken in recent days by the corruption scandal, Uribe is still expected to win a second term when Colombians go to the polls May 28.

On Thursday, the Nuevo Herald, the Spanish-language partner of the Miami Herald, reported the U.S. government had denied a visa to Montoya because officials believed he participated in money laundering operations.

"The denial of a visa can not become for Colombians a guarantee of good conduct, for there are many good people who do not have one and many others who aren't deserving of one, but posses one anyway," Montoya said.

Montoya is believed to be a close friend of the man who has been at the center of the corruption scandal: Jorge Noguera, who led Uribe's 2002 campaign in Magdalena.

In the run-up to that vote, Noguera allegedly collaborated with paramilitary groups, who are heavily involved in the drug trade, to illegally obtain as many as 300,000 votes for Uribe in Magdalena.

Uribe earlier this month said Montoya, a liquor importer, was an "honest businessman."

Noguera, Colombia's consul in Milan and previously head of the DAS secret police, returned to Bogota this week to defend himself against the charges.