The former Charlton forward has made only three league starts this season and new Bantams boss Peter Taylor sees McCammon as the man to lead his forward line at Valley Parade.
Taylor, who guided City to a 3-1 win at League Two leaders Rochdale in midweek, has already added Liverpool full-back Robbie Threlfall to his squad and McCammon will not be the last of his new recruits in West Yorkshire.
Millers boss Ronnie Moore told BBC Radio Sheffield: "Yesterday everything seemed fine.
"We had a call late on from the agent saying he'd had a change of mind and he's going to go to Bradford. It's disappointing for us."
He added: "We thought the guy would like to come and take us on to getting into the top three.
"Is it the ground? Is there some other reason why he's not come?
"It disappoints you because I know they get 11,000 or whatever it is they've got in season ticket holders there.
"Maybe he's gone for that reason."