How: Require root password instead of user password for GUI programs

["B.M." <b-misc@gmx.ch>]

Hi,

I configured my system such that some users are in group sudo, but they are
asked for the root password instead of just their user password by creating a
file within /etc/sudoers.d/ with the line:

 Defaults rootpw

This is working just fine, but for graphical applications it doesn't work: e.g.
to start synaptic I get a password prompt requiring my user password, not the
root password.

How can I configure my system such that entering the root password is also
required in these cases?

(Maybe there is something with polkit, but I couldn't figure out myself...)

Thank you.

Have a nice day,
Bernd