I don't know the difference between a liberal, a progressive, and a leftist. In my life I have been called and identified as all three. I know this isn't the point of your story, but it is implied that you do know the difference.

there's an easy way to tell the difference between liberals and leftists

In the context of the US they're 3 labels that can be applied to people on the left. Liberals and progressives are people who think everything bad in politics is caused by the right. A leftist is a person who thinks everything bad that happens in politics is caused by liberals.