"The one thing most grandparents have in common is that they have the most wonderful grandchildren in the world - so clever, so handsome, so pretty, ever so precious...your grandma and grandpa love you and have faith in you. That is your weapon! "Precious" needs to get on the phone and say, "Grandpa, Grandma, I am asking you to vote for Barack Obama. This is really important to me. It's about my future. It's about the world I will be living in. It's about the world I want for my future children. (They will love that one!) Please! Do it for me!"
Put some urgency in your voice. Sound very disappointed in them if they give you excuses. Come back again, even harder. "This is about my future - my ability to get a good job, to live a healthy life, to have the same (or even more) opportunities than you had to succeed. I have never felt more strongly about anything. I am begging you to vote for Barack Obama. I need you to do this for me!"
This is just a sample script. You know what it takes to get to them.