If Catholics in public life “openly and clearly dissent from fundamental doctrine” then they should be admonished openly and clearly in public.
Amongst other things, politics is about the market place for human ideas. If a Catholic publicly and obviously promotes dissent from fundamental doctrine, then the Church should, without hesitation, make its fundamental doctrines public and tackle the dissenter head on. In the process, the Church can demonstrate that those Catholics who dissent from fundamental doctrine are not being true to the faith they profess to be a part of and then, the obvious inference the public can make is that those very same Catholics in public life cannot be trusted to be true to anything. In other words, show them up as the hypocrites they are.
If the dissenting Catholic still persists with their obvious and public dissent from fundamental doctrine, then the Church should, without hesitation, excommunicate them. That will show the public that the Catholic Church truly stands by what it believes in.