There’s a continuing debate in English usage about when you should use who and when to use whom. According to the rules of formal grammar, who should be used in the subject position in a sentence, while whom should be used in the object position, and also after a preposition. For example:
Who made this decision? [who is the subject of the sentence]
Whom do you think we should support? [whom is the object of support]
To whom do you wish to speak? [whom is following the preposition to]
Some people do still follow these rules but there are many more who never use whom at all. Common practice in current English is to use who in all contexts. For example:
Who do you think we should support?
Who do you wish to speak to?