Final answer:
Probable cause is the standard for both warrantless searches and for a warrant to be issued, which is articulated in the Fourth Amendment that protects against unreasonable searches and seizures.
Step-by-step explanation:
The standard for conducting a warrantless search, probable cause, is the same standard necessary for a warrant to be issued. The Fourth Amendment of the U.S. Constitution protects citizens against unreasonable searches and seizures. It states that no warrants shall issue without probable cause, which must be supported by Oath or affirmation and must particularly describe the place to be searched and the persons or things to be seized. Probable cause is a legal threshold that requires a reasonable basis for believing that a crime may have been committed or that evidence of a crime can be found in the place to be searched.
When a warrant is needed, law enforcement officers must demonstrate to a judge or magistrate that there is probable cause to believe a crime has been committed. However, the Supreme Court has ruled that there are certain situations, such as when searching the passenger compartment of a car or when people are entering the United States, where a warrant is not required. In such cases, the exception to the warrant requirement still relies on some level of probable cause or its equivalents, like exigent circumstances or plain view doctrine.