WASHINGTON (MarketWatch) - Under pressure from both Democrats and Republicans, the head of the U.S. Office of Personnel Management resigned on Friday after a series of breaches that exposed the Social Security numbers of millions of federal employees. OPM Director Katherine Archuleta reportedly stepped down amid rising criticism in Congress. More than 20 milllion Social Security numbers were compromised, possibly by Chinese hackers. Archuleta initially resisted calls to resign and the White House reiterated support for her as recently as Thursday.