Stopping Lawless Actions of Politicians Act of 2018 or the SLAP Act of 2018 This bill amends the federal criminal code to make it a crime for a state or local official to knowingly release from custody an individual who is an alien subject to a detainer. It imposes criminal penalties—a fine, a prison term of up to five years, or both—on a violator. A state or political subdivision that complies with a detainer is deemed to be an agent of the Department of Homeland Security and is authorized to take actions to comply with the detainer. The bill limits the liability of a state or political subdivision, or an officer or employee of such state or political subdivision, for actions in compliance with the detainer.