Clinton, for example, stole the crime issue from Republicans and it was difficult for Republicans to combat him on it. Davis attempted to do the same thing, and in 1998 it worked. Even though Lungren was the Attorney General, Davis worked double time to secure the law enforcement endorsements by promising the moon and the stars.
But crime is going up, and people don't like that. Crime is on the rise as an important issue, though it hasn't come up as far as education. Davis losing the COPS endorsement IS a big deal for a lot of reasons: he no longer has the lock on crime as an issue, and money.
Hope that helps.