By Isabella Cota

MEXICO CITY (Reuters) - Mexican drug cartels paid $4.5 million in bribes to buy protection and political favors in a state run by the country's main opposition party, U.S. court documents said on Friday, as the party leads polls to win the presidency in July.

The money-laundering case in Texas charges Antonio Pena, arrested on Wednesday, with funneling cash from the feared Zetas cartel to officials in the state of Tamaulipas, according to documents from the U.S. District Court in San Antonio, Texas.

A sworn affidavit from an undercover agent at the U.S. Drug Enforcement Administration names former Tamaulipas Governor Tomas Yarrington of the Institutional Revolutionary Party (PRI), as having a direct personal relationship with Zeta leaders.

A DEA undercover source "described Antonio Pena as a conduit between Mexican politicians, in particular Tomas Yarrington, and Zeta members Miguel Trevino and Heriberto Lazcano," according to the documents, available on U.S. court database PACER.

Lazcano and Trevino are the leaders of the cartel, which is notorious for decapitations and kidnappings across Mexico.

Yarrington, in office from 1999 to 2005, is under investigation separately in Mexico along with two other ex-governors of the state, which the PRI has ruled since the party's foundation.

The centrist PRI has said the probe is politically motivated ahead of the hotly contested July 1 presidential vote.

The federal attorney-general's office confirmed the investigation but declined to give further details. Mexican media have reported authorities are searching for evidence the politicians are linked to money laundering.

The accusations have given President Felipe Calderon's ruling conservative National Action Party, or PAN, fresh ammunition against the PRI, which ruled Mexico for seven decades until it was ousted in 2000.

Yarrington did not immediately reply to requests for comment. He said via a Twitter account on January 30 that his name had appeared on an alert blocking his travel outside of Mexico.

"I hope that the authorities clarify the motive and scope of this action," his Twitter account said.

The PRI is hoping its youthful presidential candidate Enrique Pena Nieto, who has a big lead in opinion polls, will bring a new face to the party, still remembered by some Mexican voters for corruption and vote buying during 70 years in power.

HORRIFIC VIOLENCE

Members of Calderon's PAN have accused the PRI of negotiating in the past with organized crime, a charge the party vehemently denies.

PAN party chairman Gustavo Madero responded to the Yarrington report by urging a thorough investigation of alleged links between the PRI and organized crime.

"Our concern isn't political posturing, it's an appeal to the conscience of those governing to think again, because this is putting the lives and well-being of all Mexicans at risk," he said in a televised statement.

The PAN's popularity has suffered since the government launched a frontal attack on the cartels five years ago. Since then, more than 47,000 people have died in Mexico as turf wars between the gangs over lucrative smuggling routes intensified.

The Zetas, who began as the armed wing of the Gulf cartel but split off from their former employers, have been blamed for some of the most horrific acts of violence, including the massacre of 72 migrants in Tamaulipas in 2010.

In the Texas court case document, Yarrington is mentioned several times.

"This relationship started with the election of Governor Tomas Yarrington and continued with the placement of other PRI candidates in government positions throughout Tamaulipas who could ensure favorable protection for the cartels," the testimony said.

The DEA agent says Yarrington met with Pena on various occasions in Nuevo Laredo, Mexico, and the United States.

The testimony also cites another confidential source who said he delivered $500,000 in drug proceeds from the Gulf cartel to Pena when he was an associate of the mayor of Nuevo Laredo in 2002.

John Ackerman, a political analyst at the National

Autonomous University of Mexico (UNAM), said the government should have begun probes into the ex-governors much earlier.

It chose not to because the PAN needed opposition support in Congress due to its lack of a majority, he added.

"This is not a witch hunt, this is the reality: governors and government officials in this country are involved with drug dealers," he said. "They are not clean, neither those from PRI nor the PAN."

(Reporting by Isabella Cota; Editing by Mica Rosenberg, Krista Hughes and Philip Barbara)