Most people don't do extensive (or any) research before voting. They choose a candidate based on party affiliation or the information on TV. So for that majority of people, they will vote for a candidate whether or not the candidate is of sound mind. They assume other people have done the due diligence.
On the other hand, you have the parties themselves. The Democratic Party would rather have a senile Democrat than a non-senile Republican. And the Democratic Party is itself strongly influenced by other Democrat politicians who may even appreciate a senile coworker since that coworker can be more easily manipulated. So they have no incentive to risk losing that by suggesting or supporting a different Democratic candidate.
That in turn means new Democrat candidates will struggle to get the amount of support or funding which is necessary to publicize oneself enough that the complacent members of the public mentioned earlier could vote for them.