Yep - that's my favorite, although I prefer to draw 'em out a little more and give 'em a little more rope. So I ask, "Towards what, exactly, are you progressing?" Often as not, there's no coherent answer. But if they answer something like "social justice," then I ask them what that means. That ususally leads to some stupid and indefensible definition. To which I raise an eyebrow and ask, "At whose expense?"
Yes, I know - it's something akin to pulling wings off flies, but it's good sport.
Next question:
"And just how are you going to make people pay for that?"
A:Taxes
"So you condone using the legal lethal force of government to force people to pay for your pet ideas? Sounds more like fascist totalitarianism than 'progressivism'."