WITH MITT Romney’s candidacy for president, the Mormon church approaches an epochal moment in its deep engagement with American politics. The nation, too, is at a threshold - entering, perhaps, a more spacious public understanding of many once-marginal groups. In the Mormon case, it’s been a long time coming. Romney may be a front-runner for the Republican nomination, and his father George may once have been a serious candidate for president, but the first Mormon to run for president was the first Mormon himself.