I'm sorry...I'm having a little difficulty placing the obliteration of "undesirable" people and characteristics, as in Nazi Germany, in the same category with restricting voting rights.

I believe that there is nothing inherently wrong with raising the bar for voting. Most people see nothing wrong with placing restrictions on other privileges, like driving, and many people have no trouble in placing massive restriction on the Constitutionally-protected right to bear arms. In our current society, where our government has the potential for nearly limitless power over the citizen, a voter can do a lot more damage than a handgun owner.

However, as you rightly point out, and as I have admitted before, selecting exactly what characteristics make for a good voter is a non-trivial problem, and I have neither come up with or heard any sensible and workable ideas consistent with the Constitution and individual rights.

It is also becoming obvious that the concept of having anyone with a pulse able to cast a vote isn't all that great either, but until something better comes along, we need to live with it. And since the politicians benefit by having voters as uninformed, stupid, and easily-swayed as possible, there's no real incentive from above to change either.