The reason for Republican resistance to automatic voter registration, is fear of losing majority in state governments throughout the country.
As for the need. A great number of Republican majority state legislatures throughout the country have closed many voter registration offices in selected areas of their state. In some cases an individual would have to go to the county seat to register to vote. For some that could be over fifty miles. A it may surprise some but there are a fair number of people who do not own a motor vehicle.
So yes there should be automatic voter registration at one’s eighteenth birthday. An voting should be a requirement of citizenship. If you want to live within a government that is of for and by the people you must participate.