Many Democrats don't support the Immigration Bill (Hell, polls show 70%-80% are against Amnesty).
By going out and giving speeches against the Bill, Fred would draw a distinct line between the Republican candidates and the Democrats (All of whom support Amnesty). Fred would also distance himself from the current Bush Administration, which favors Amnesty, and has support of only 29% of the American public.
In short, this could be called "Amnesty Triangulation."
I agree. This will be the issue that decides the next election. He would be the perfect man to lead the revolt, because he spent 8 years in the Senate and knows how to maneuver to get it killed.