We just had a Republican primary this week in Ohio for a safe Congressional seat. Out of 11 Republican candidates, none chose to make it an issue. Illegal immigration is one of those things that everyone says they're against, but only a very tiny minority rank it as one of their top concerns.
Any candidate who screams like Howard Dean that "They're coming here to kill you, and you, and me, and my grandchildren," will be recognized for what he is.
Issues affected by illegal immigration are many times at the top of the list of concerns by voters. Voters often express concerns about crowded schools, congested freeways, crowded emergency rooms, crime rates, available jobs, communicable diseases, etc. All of these can be and are affected by immigration, both legal and illegal.