With the repeal of all poll taxes, the United States has attained as mature a form of democracy as can be in keeping with it's federal organization.
The U.S. Senate may be regarded as non-democratic in the sense that as of 1998, 20% of the people elect half the Senate, but abolition of the Senate would change the United States of America into the Nation of America, or the Republic of America, or some such thing.
Demographers who study population growth, migration, and immigration tell us that this phenomenon will become more pronounced in the future and that we could in our lifetime see 10% of the people electing half the Senate.