That is the down side of ANY and ALL of our Constitutional rights, they are given to all at birth (or at least citizens upon attaining adult age). Once you assert that "training" or "qualification" may be added to get them then one may ask why allow voting by morons? Much more harm, to our rights and freedom, can be done by passing restrictive laws and ceding ever more control/power to the gov't by voting for (and by) morons, than can be done by any single individual with a gun. Many elections are decided by less than 5% of the voters. Calls for adding the requirement of a simply presenting a valid, state issued, photo ID to vote are often rejected, much less adding any tests of voter competency, e.g. naming the current US representatives, two Senators and one House representative. Those that are very quick to demand "strict qualification tests" for getting one's 2A rights seem to be the very same folks that would deny any "reasonable restriction" being paced on the right to vote. I will take your "public safety" concerns seriously when you pass and enforce laws to permanently lock up all repeat felony offenders (and the criminally insane) and require only citizens with valid, state issued, photo IDs showing current residency be allowed to vote (only once and only as themselves) in a given election.
