The myth, which liberals like myself find tempting, is that only the right has changed. In June 2015, we tell ourselves, Donald Trump rode down his golden escalator and pretty soon nativism, long a feature of conservative politics, had engulfed it. But that’s not the full story. If the right has grown more nationalistic, the left has grown less so. A decade ago, liberals publicly questioned immigration in ways that would shock many progressives today.