New York City mayor Bill de Blasio’s approval ratings have slipped below 50 percent, making it possible he could face a serious primary challenge in 2017. But New York’s pro–de Blasio city council may have found a way to stop that cold: A majority of the council supports giving more than 1 million non-citizens full rights to vote in local elections. That’s one in five adult New Yorkers. Mayor de Blasio’s approval ratings are much higher among minorities than with whites, who oppose him by a two-to-one margin. Giving non-citizens — most of whom are minorities — the vote could...