LAS VEGAS—It’s Donald Trump’s world. Mitt Romney just diminished himself in it. That’s the lasting impression left by Thursday’s media frenzy in Las Vegas, where Romney steered his increasingly momentous drive for the Republican presidential nomination into the cartoon arms of a clearly delighted Donald. Endorsements come, endorsements go — and in most elections, they don’t mean all that much. Especially coming from self-aggrandizing hype factories on the sidelines. But in making the pilgrimage for what was essentially a ring-kissing ceremony inside Trump’s gilded hotel on the Vegas strip, Romney appears to have gone carnival barking up the wrong tree....