APRIL 7--When friends and family members gathered recently at the White House for a private celebration of Michelle Obama’s 50th birthday, one of the invited partygoers was a former paid FBI Mafia informant.That same man attended February’s state dinner in honor of French President Francois Hollande. He was seated with his girlfriend at a table adjacent to President Barack Obama, who is likely unaware that, according to federal agents, his guest once interacted with members of four of New York City’s five organized crime families. He even secretly taped some of those wiseguys using a briefcase that FBI technicians outfitted with a recording device.The high-profile Obama supporter was also on the dais atop the U.S. Capitol steps last year when the president was sworn in for a second term. He was seated in front of the chairman of the Joint Chiefs of Staff, two rows behind Beyonce and Jay Z, and about 20 feet from Eric Holder, the country’s top law enforcement officer. As head of the Department of Justice, Attorney General Holder leads an agency that once reported that Obama’s inauguration guest also had La Cosa Nostra contacts beyond Gotham, and engaged in “conversations with LCN members from other parts of the United States.”The former mob snitch has become a regular in the White House, where he has met with the 44th president in the East Room, the Roosevelt Room, and the Oval Office. He has also attended Obama Christmas parties, speeches, policy announcements, and even watched a Super Bowl with the First Family (an evening the man has called “one of the highlights of my life”). During these gatherings, he has mingled with cabinet members, top Obama aides, military leaders, business executives, and members of Congress. His former confederates were a decidedly dicier lot: ex-convicts, extortionists, heroin traffickers, and mob henchmen. The man’s surreptitious recordings, FBI records show, aided his government handlers in the successful targeting of powerful Mafia figures with nicknames like Benny Eggs, Chin, Fritzy, Corky, and Baldy Dom.