Surprised that the Democrats stuck to their guns on the shutdown, even opening themselves up to the charge that they are “far more concerned with illegal immigrants” than with the military and border security, as President Trump put it in a Saturday morning tweet?

Don’t be. It’s been coming for a while. And while liberals should cheer it in the short term, all citizens should be aware of the potential long-term stakes for the republic of the Democrats becoming more a mirror image of the Republican Party.

First, let’s establish that we agree that this is not your father’s Democratic Party. Democratic leaders of a generation ago would have been paralyzed with fear at being branded the way Mr. Trump described them this weekend. Part of the change is grounded straightforwardly in public opinion. Twenty years ago the polling on Dreamers would surely have been very different; today, poll after poll shows 80 or 90 percent of respondents supporting the Democratic position that they be permitted to stay and given a path to citizenship.

But another part of the change is that the Democrats, little bit by little bit, are becoming more like the Republicans. Critics of our polarized conditions who have blamed both parties equally don’t understand that the parties are fundamentally different creatures. The Republican Party is a movement party. The Democratic Party is not.