Over the last decade, former President Bill Clinton has raised more than $500 million for his foundation, allowing him to build a glass-and-steel presidential library in Little Rock, Ark., and burnish his image as an impresario of global philanthropy. The foundation has closely guarded the identities of its donors — including one who gave $31.3 million last year. Now, the secrecy surrounding the William J. Clinton Foundation has become a campaign issue as Senator Hillary Rodham Clinton seeks the Democratic presidential nomination with her husband as a prime source of strategy and star power. Some of her rivals argue that...