[This open letter is being published simultaneously by The Bulwark and the New Republic.]We are writers, academics, and political activists who have long disagreed about many things.Some of us are Democrats and others Republicans. Some identify with the left, some with the right, and some with neither. We have disagreed in the past, and we hope to be able to disagree, productively, for years to come. Because we believe in the pluralism that is at the heart of democracy.But right now we agree on a fundamental point: We need to join together to defend liberal democracy.Because liberal democracy itself is...