It is now. Under the Fourth Amendment, which guards against unreasonable searches and seizures, law enforcement personnel needed “probable cause” to stop and search someone. That is, they need good reason to believe a person has committed or is about to commit a crime.
But a 1968 US Supreme Court decision, Terry v. Ohio, lowered that benchmark by allowing law enforcement to stop and search people in a public place if the officer “reasonably suspects” that someone is about to commit a crime. Reasonable suspicion alone is not grounds for arrest, as probable cause is, but incriminating evidence found during a stop and frisk may be grounds for arrest.
The problem is that “reasonable suspicion” is more vague than “probable cause,” leading some opponents to argue that law enforcement officers use a person's race to determine reasonable suspicion, and therefore, perform stop and frisks.