There's an article in Forbes pointing out that engaging in that line of attack could backfire.
If the Democrats want to change the conversation on this, they ought to hammer home the fact that the Republican mantra that employment eliminates dependency and precludes requiring welfare benefits holds absolutely no water in the 21st century. The dichotomy of a person working and being self-sufficient or a person not working and getting money from the state is a false one. The real choice is between working and needing a little bit of public assistance or not working and needing a lot more public assistance. We can tinker with the minimum wage, but it ultimately boils down to a question of how many poor people you want to be employed versus unemployed and the degree to which their income is going to be derived from transfer payments.
I'm sorry, progressives, but if you have a McDonald's franchise with 20 people making $8 an hour, and you raise the minimum wage to $15 an hour, you're not going to have 20 people making $15 an hour. You're going to have less than 20 people making $15 an hour and the remainder not working at all and needing even more state assistance than they would have needed if they worked at McDonald's for $8 an hour. Conversely, Republicans, the McDonald's owner is not going to go all Ayn Rand and shut down his restaurant and go live in Galt's Gulch.
The problem with the analogy is that Wal-Mart does more than just pay their employees crap, they hurt other businesses (and not just other retailers, many of the companies that make the stuff they sell have been deeply hurt by Wal-Mart's demands to set their prices) and drive down wages in general. McDonald's isn't exactly a stellar company either, but no five star restaurant is going to worry about going out of business because of competition if a McDonald's opens up across the street from them, even though their food is obviously going to be far more expensive.
What's also kind of funny is that Adam Smith actually warned about Wal-Mart-esque practices and was actually in favor of regulation to prevent such things since a monopoly isn't any more of a free market than socialist state control. Of course how many teabaggers are aware that Smith was also supportive of labor unions?