Is it more democratic for a group of people to exclude another group of people from voting than it is for one individual to exclude a group of people from voting? Either way, a proportion of the people of a country are being excluded from having a say in how the country is run. This is not republican - it's not the the way that America was intended to be.
If there will be a compromise in the American ideal either way, between changes in the organisation of society or becoming something that is not a republic, why should the priorities that one group of people determines for directing freedom have greater power to determine that sort of change than the priorities of any other group of people?
Why is a country that excludes people from voting on the basis of their political convictions more free than a country that excludes people from voting on the basis of their religious beliefs, or their sex?
Was your remark that groups of people should be excluded from voting intended seriously?