They aren't attempting to nullify federal law. They are trying to change state law that applies to the same offenses. The feds can continue to do as they please, but the state doesn't have to waste it's time and money finding, arresting, prosecuting, incarcerating, and probating these particular criminals.

If is passes, I wonder how long it will be until the federal government starts withholding federal funding for education, transportation or law enforcement.