By Marty Graham

SAN DIEGO (Reuters) - A former head of the U.S. Border Patrol union has been charged with misusing union and government funds to enrich himself, maintain a mistress in Chicago and pay for pornography, a federal indictment said.

The indictment handed down by a federal grand jury late on Thursday charges Terence "T.J." Bonner, who for 22 years served as head of the National Border Patrol Council, with one count of conspiracy to commit wire fraud and 11 counts of wire fraud over a period of five years.

"Siphoning hundreds of thousands of dollars from hard-working fellow Border Patrol agents, many of whom put their lives on the line every day to protect this country, is a particularly troubling form of corruption that must be addressed," San Diego U.S. Attorney Laura Duffy said in a statement.

Bonner, whose last two-year term as president ended in March 2011, will be arraigned on Monday and has not yet entered a plea. According to court records, he has not yet designated a defense lawyer.

"I never cheated the government nor the members of the union of a penny during my two decades of service. I consistently spent at least 60 hours a week on union business. At the end of this case, it will be clear that under any fair accounting, I am actually owed well in excess of $100,000 for time spent and expenses incurred," Bonner, 59, said in a statement.

The union's current president, George McCubbin III, told Reuters that throughout the two-year investigation, Bonner maintained he'd done nothing wrong and that the investigation was merely "a power grab" within the union.

"When it became obvious what he'd done, we asked him to step aside," McCubbin said. "He took the view he didn't do anything wrong."

According to the 10-page indictment, Bonner was able to write himself checks from the union fund that were co-signed by the union's treasurer, while he submitted false claims for business travel, overtime and weekend wages, and for reimbursement for credit card and other expenses. Continued...