When I tried to start partitionmanager (KDE Partition Manager) on Fedora 26, it would always ask for the root user’s password, despite the fact that the root user was disabled in my setup.
The dialog box that popped up looked like it might be kdesu. kdesu,. a.k.a. KDE su, is a KDE version of su that allows a process to be run as another user, like root. When I tried running kdesu separately it asked for root’s password again. So I knew the problem lay within kdesu, not partitionmanager.
The ever-helpful Arch wiki contained just the golden nugget I was after: a command that would configure kdesu to use sudo:
kwriteconfig5 --file kdesurc --group super-user-command --key super-user-command sudo
This fixed the problem for me, as sudo requests your password, not root’s. Whether you’re allowed to run the command depends on the contents of the system’s sudoers file. Look that file up if you continue to have difficulty.