Mark McNally, in the comments over there says this:
Actually it is part of the wider rule: Any issue the helps Republicans is an issue unworthy of discussion and divisive: see abortion, marraige, illegal immigration, crime, terrorism, etc.That is exactly right... Instead of addressing the actual issue and the problems caused by illegal immigration, our friends on the left side of the Ohiosphere and beyond are going to cry "Foul!" every time. Why? Because Mark is right...this isn't an issue that the Democrats have an answer that the American people will buy and they want to minimize the danger. The problem with that strategy is that the issue is not going to go away tht easily...
Any wedge issue that benefits Democrats is seen as a good thing. Example: stem cell research, minimum wage, throwing more money at feckless teacher unions, etc.