“Whom” is correct here. It is the “object” form, while “who” is the “subject” form. This means you use it after a preposition, as J. Rios says, but also after a verb when it is the object of the verb. Here, “hire” is a transitive verb that takes a direct object, which in this sentence is “(the candidate) whom.” However, in common speech, people tend to use “who” in all cases.

1. It is NOT true that "whom always needs a preposition". 2. The pronoun in question is the object of the verb 'believe', so it's correct to use 'whom'. 3. 'Whom' is correct, but not common. In everyday speech, it is acceptable - and more usual - to use 'who'. 3. Note that you can also omit this pronoun entirely. In speech, this is often the most natural option. We might say "They say that you must hire the candidates you believe can hit the ground running" and it would be clear and correct. We tend to include the pronouns more in written English.

Thank you.

Believe Garry. If you'd say "Him" use "whom" If you'd say "he" use "who". I'd hire him. Whom would you hire? He's the one. Who is the one?

Thank you very much. But for all I know, reletive prons should be understanded in its own clause, rather than the main clause before, that's to say, '... the candidates WHO can hit the ground running', [subject to 'can'] or in another example, 'Desperate for money, she called her sister, WHOM she hadn’t spoken to in 20 years'. [object to 'spoken to' ]