There is nothing that you can't express in a kind and constructive manner. Speak objectively, keeping judgments, blame and "you" statements out of it.
"It's been difficult to add new features to the code base because it's grown so much and I think we need to consider a serious refactoring project. The best approach in my opinion is to take the lessons of the past and start over for X and Y reasons."
Will everyone agree with you? Maybe not. Will it work? Maybe it won't. But that's also true if you say "your code sucks," and you won't have to be bogged down in savage internecine code warfare.
