Dayton's has had the bigger drop, from 58% to 43% since the January 2004.
Coleman's has dropped from 54% to 47% since January 2004.
Obviously, this is a bigger concern for Dayton. Not only is he up for re-election sooner, but historically, Star-Tribune polls lean about 5-8% Democrat anyway.
Charlie Cook, at the independent Cook Report says that Dayton is now the most vulnerable incumbent now for 2006 (even more than Santorum, Chafee or Nelson (FL)).
I tend to agree with his analysis.