Dorgan said Roberts misled him into believing the chief justice would not be on the far right.
"The representations that were made and implied turned out not to be accurate," Dorgan said, referring to Roberts.
Rockefeller said the chief justice's performance on the court does not reflect what Roberts said at confirmation hearings.
Sen. Kent Conrad, D-N.D., who voted to confirm both Roberts and Associate Justice Samuel Alito, said he would do things differently if he knew then what he knows now.
"I feel I was misled by some of the previous candidates for the court," Conrad said.
The Democrats' comments come as some centrist Republicans consider whether to break with their party and support Kagan.