A long-term loan deal or a free transfer switch are in the offing for the 31-year-old former Blackburn, Derby and Barnsley star.
Bantams chairman Gordon Gibb confirmed the interest in the player, telling the Yorkshire Post: "Portsmouth have been in touch and we are talking over the finer details.
"This is a financial decision on our behalf, but hopefully some of the money this deal would free up will allow the manager to bring in another striker to replace Ashley."
Fratton Park boss Redknapp added: "He's only one of several players I'm interested in, but I do like Ashley Ward."