(By Larry J. Sabato, university professor of politics and director of the University of Virginia Center for Politics, Kyle Kondik, managing editor of Sabato’s Crystal Ball and Geoffrey Skelley is associate editor at Sabato’s Crystal Ball)  The two factors that might be most vital to Hillary Clinton’s chances—the identity of the eventual Republican nominee and the standing of President Obama—are elements over which she has little influence. It’s the second factor, Obama’s job approval rating, that is especially critical. And the state and...