In modern democracies, universal suffrage is seen as the corner stone of liberty and proper political conduct. Its universality is congruent with the universality of its acceptance. Therefore, a convincing argument against it must proceed from premises that are accepted with that same universality. Let us begin.
The first point is that no one endorses, defends or even wants universal suffrage and the reason why undermine the degree of universality suffrage now has.
That universal suffrage does not actually exist anywhere is seen in a number of rules that restrict access to voting, among others age and sanity. No clear minded person argues for the right of a toddler or even an adolescent to vote. This becomes clearer if we consider what would constitute a good rational vote.
Voting is the final act in the course of a communal participation in practical rationality. That is there is some deliberation about some such course of action. In what manner in which voting is related to this course of action is not relevant, except in so far as it concerns the use of practical reason.
Young children, the insane and others are generally no capable of the use of practical reason in the first place. Those older children that are capable of its use lack the knowledge and experience to use their practical reason in such a way as to arrive at a truly reasoned (i.e. not emotive) practical conclusion. Therefore, they are universally restricted from access to voting.
If we consider the reason we restrict access to voting for children, we should arrive at the conclusion that very few people should be permitted to vote on issues of a national scale.
Consider the several issues that have arisen during the election from the economy, to foreign policy, to domestic policy. Within each of these broad categories there are innumerable sub-issues with their own intricacies. In the election, we are called to make some sort of practical judgement on these issues in such a way as to arrive at a conclusion regarding who to endorse or support.
I propose that the great majority of people do not possess the knowledge, training or capacity to make practical judgement with respect to the relevant national issues. This is demonstrated most poignantly by the reasons that people give for voting this way or that. With no rational basis in reality to make judgments, the great majority of people rely simply upon an act of emotion. This particular policy appeals to me, my sensibilities, ideology or some other commitment that may be more or less rational. However, the judgement in itself remains emotive rather than rational.
Therefore, in so far as voting is the final expression of practical rationality engaged in by a community, voting on national issues is generally irrational, emotive and hence immoral.