At home, Julián Castro's been spending more time reading and watching television in Spanish, trying to get his speaking skills up to speed. On the job as Housing and Urban Development secretary, he's been carefully working the levers in Washington, with coaching from Bill Clinton and a twin brother who's a popular and up-and-coming congressman himself. Starting Saturday, he'll be out on the trail for Hillary Clinton in in Nevada, Iowa and Maine. He's plotted his rise carefully, studying and strategizing with a clear goal in sight. But if Clinton picks him to be her running mate, it'll be more...