Redknapp wants chairman Daniel Levy to back him in the transfer market to show Modric the club means business.
Redknapp told The Sun: "Luka wants to be successful and if we can improve the squad then he can achieve that with us.
"I don't envisage a problem with Luka. You could not meet a more professional man or a nicer person.
"It is so important for the club to keep him. I think we have to show him our ambition, which we will do.
"If we can add one or two good players to our squad there's no reason we can't have another great season. So Tottenham could be a great club for him to stay with for the next few years.
"We want to get back in the Champions League and we need him to help us do that."