LAKE FOREST, Calif. — When John McCain and Barack Obama appear on the same stage today, they will vividly demonstrate the reach that has made the Rev. Rick Warren among the most significant evangelists of his generation. He's a megastar who leads the nation's fourth-largest church and reaches thousands of ministers through the Internet and crusades against poverty and AIDS. That globe-trotting work and his successful book — "The Purpose-Driven Life: What on Earth Am I Here For?" — puts him at the vanguard of a movement that inspires young, socially conscious Christians. But his willingness to soft-pedal political issues...