One-party rule is hurting America’s big cities. But not everyone thinks it’s a problem. Bruce Katz of the Brookings Insititution, for example, has called for a national Metropolitan Party that advocates for urban interests. In truth, the country already has a Metropolitan Party, and it’s called the Democratic Party. Cities overwhelmingly went for Hillary in 2016. And since the election, national politics is moving even more toward a big-city alignment against the policies of President Donald Tru…