Good question.
The Incumbent Rule states that an Acceptable Challenger will take 80% of the Undecideds from a non-wartime President seeking re-election.
Eastwood is one of the most beloved and respected figures in America, not for any reason other than being a great actor/director, but there it is.
If any Undecided had any reservations about Mittens as an Acceptable Challenger, Eastwood's blessing could quite easily move Joe Six-pack off the dime and into the voting booth. Easily.
I guess there’s not much harm it can do, but I can’t see it helping much. Maybe some seniors will identify with Eastwood. Just hope it doesn’t come off as lame.