Earlier this month, Mayor Lou Barletta of Hazleton, Pennsylvania - currently running for Congress - invited Barack Obama, Hillary Clinton, and John McCain to his city to have a debate of some kind, saying in part:

If you are interested in addressing the issue of illegal immigration, then you must come to our city to hear our stories and learn how cooperation and effective leadership can work to correct the problem and how outside forces -- primarily in Washington, D.C. -- continue to work against our citizens

Now, per this, comes the word that John McCain has politely declined the invitation. Scheduling conflicts or something. The other two haven't responded at all.