The Fourth Amendment reads as follows: "The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized."
This one, most people seem to agree on, for the most part. People do not want to be randomly searched, and they agree, generally that this is bad. There was a policy, I believe in NY, several years ago that met with controversy— stop and frisk. Many people were on board with stop and frisk, because it could prevent crimes from occurring. I don't know the statistics on this, so that is not what I am debating. The fact is, regardless of the effectiveness, it violates the constitution. It is unreasonable to search an individual walking down the street. The amendment says that citizens cannot be searched, nor their property taken by law enforcement without a warrant. We can't search people because we want to, we think they don't belong in an area, or it might stop a crime. We have to follow the constitution and obtain a warrant. There is the exception of probable cause. If an officer stops someone and they are truly acting suspicious or if they feel there is probable cause that a crime is occurring, they may be permitted to search you anyway. This is where the word unreasonable comes into play. Unreasonable means that you cannot be searched willy-nilly, but you may be searched if there is a good reason. If you appear to be hiding something or trying to obstruct the officer's view of something, you may be searched.
An officer may not enter your home and search it. They may not stop you on a traffic stop and just search you. They must a.) ask permission or b.) have probable cause. If an officer knocks on my door tonight and asks me if he can search my home, I am well within my constitutional right to say no. That said, the officer will often attempt to get a warrant at that point, making the search legal. However, if he has no justifiable reason, a warrant would be conceivably more difficult to obtain. The same is true for a traffic stop or for being stopped while walking down the street.
No one wants to be unnecessarily searched, and the constitution protects that.