I think using the word "liberal" is perfect. He forces the Dems who are offended to admit they are liberal first, then it forces them to explain why they are not what Rove says they are. Its a pretty tall order for a bunch of phonies trying to paint themselves as moderates.
It also has the added benefit of ensnaring the Liberals hiding out in the Republican party without using their names that are sharing in the mantra of the Dem party.