Across the political map this year, Mr. Sanders’s diagnosis of his former party appears indisputable: In off-year elections from Mr. Sanders’s California to New York City and New Jersey and the increasingly blue state of Virginia with its crucial suburbs of Washington, D.C., the Republican Party’s feeble appeal to the country’s big cities and dense suburbs is on vivid display... The realignment of national politics around urban-versus-rural divisions has seemingly doomed Republicans in these areas as surely as it has all but eradicated the Democratic Party as a force across the Plains and the Upper Mountain West. At the national level, Republicans have largely accepted that trade-off as advantageous, since the structure of the federal government gives disproportionate power to sparsely populated rural states. But the party’s growing irrelevance in urban and suburban areas also comes at a considerable cost, denying conservatives influence over the policies that govern much of the population and sidelining them in some of the country’s centers of innovation and economic might. The trend has helped turn formerly red states, like Georgia and Arizona, into purple battlegrounds as their largest cities and suburbs have grown larger and more ethnically mixed.