Bare minimum, if at any point you've expressed as a politician negative views towards democracy or any core philosophical element of democracy or indicates a desire to remove or destroy it, you should be bared from running for office.
Any politician who expresses positive feelings or views of historical parties who dismantled or destroyed democracy should also bar them from running.
You wouldn't even have to use the word "fascism". Just create a law that establishes that its illegal to dismantle, weaken, or destroy democracy and that even conspiring or openly stating the criminal threat of doing so is also a crime and on top of whatever criminal penalties it also prevents you from running for office as punishment.