Bernie Sanders is on a roll. He's won the last two Democratic primaries and stands a good chance Tuesday of adding Oregon and perhaps Kentucky to his pile of victories. Yet Hillary Clinton is likely to continue her seemingly unstoppable march to the party's presidential nomination. How can that be? It's not a conspiracy, as some angry Sanders backers suggest, a result of dark magic or a wrinkle in the time-space continuum. Rather, it's the rules that Democrats play by -- rules that now work to Clinton's advantage, even as they thwarted her candidacy eight years ago, when she lost...