Personally I would go for a small series of and/ors. I don't really think it's a question of "responsibility", more of a "Stake in the future". I would say if you tick one of these boxes, you are allowed to vote. Have biological, or adoptive children(legally dependents), own a business (said business would be your main form of income), own land (This land would be worth atleast 1/4 of the middle class tax bracket's income to represent a significant amount of money), or have served in active duty(you layed down your life, you deserve a vote). Ontop of this, I would only make it mandatory for federal positions of power that you tick two of these boxes(That ways even by the "Easiest" standard you would have to own land and have children). It would be so that the average person could have more say in local and state held positions, while the more "resposible" could vote nation-wide. One caveat would be that the standard for running for these position would not change. This would result in people running that cannot vote for themselves.