The Fourth Amendment Applies Before the Case Even Reaches Trial
Police need a lawful basis to stop a vehicle, prolong a detention, search a person or property, or make an arrest. When evidence is obtained in violation of those constitutional protections, a motion to suppress can ask the court to exclude it — and in some cases, suppressing key evidence leads directly to a reduced or dismissed charge, because the fact that police ultimately found something incriminating doesn't make an unlawful stop or search constitutional after the fact.
Traffic Stops, Vehicle Searches, and Consent
A stop must be supported by probable cause or reasonable suspicion, and even a lawful stop cannot be prolonged beyond its original purpose — waiting for a drug dog, for example — without independent justification. Vehicle searches often rely on the automobile exception or consent, and consent has real limits: it must be voluntary, its scope can be limited or withdrawn, and one person's consent doesn't always authorize a search of another's belongings.
Warrants, Cell Phones, and Statements
A search warrant can still be challenged — for insufficient probable cause, stale information, an unreliable informant, or a search that exceeded the warrant's scope. Cell phones generally require their own warrant even after a lawful arrest. And statements obtained through custodial interrogation without required Miranda warnings can be suppressed, along with evidence that flows from an earlier constitutional violation under the "fruit of the poisonous tree" doctrine.