Almost no elected Republicans support giving gays the right to marry. The party’s influential social-conservative wing sees “traditional marriage” as a defining issue. And while most major Democrats are rushing to embrace same-sex marriage, none of the most prominent potential Republican presidential candidates have taken that step.But a powerful group of Republican donors, who see the GOP’s staunch opposition to gay rights as a major problem, is trying to push the party toward a more welcoming middle ground — where candidates who oppose marriage rights can do so without seeming hateful.The behind-the-scenes effort is being led largely by GOP mega-donor...