Long written off as too late to matter, California's Republican primary June 5 may turn out to matter a lot. A new Field Poll shows former Pennsylvania Sen. Rick Santorum just six points behind front-runner Mitt Romney, the former governor of Massachusetts. California's mother lode of 172 delegates has long been considered Romney's firewall against a contested or even brokered national convention in Tampa, Fla., in August. A contested convention is the ultimate nightmare scenario for the GOP, which has savored this year as its best chance since Jimmy Carter to deny a second term to an incumbent Democratic president."It's...