Former House Majority Leader Dick Armey (R-TX) regrets voting for the Iraq War resolution in 2002:

The resolution was a resolution that authorized the president to take that action if he deemed it necessary. Had I been more true to myself and the principles I believed in at the time, I would have openly opposed the whole adventure vocally and aggressively. I had a tough time reconciling doing that against the duties of majority leader in the House. I would have served myself and my party and my country better, though, had I done so.

I think that's farther than Sen. Clinton has been willing to go.