Pronouns! Whom is correct here.
Who / whom works like He / him —
“The Trump administration attempted to label him a terrorist.”
“The Trump administration attempted to label whom a terrorist?”
“He is the person whom the Trump administration attempted to label a terrorist.”
On direct vs indirect object, I’ve heard that before but it doesn’t make sense. None of the other pronouns in English work that way.
Best supporting authority I could find in 15 seconds of looking was Merriam Webster.