limit the number of years a senator can serve to 12 years.

Currently there is no limit to the number of years a United States Senator may serve. This creates an environment of careerism in the US Senate (all houses). Senate service was originally envisioned as part-time service to your country, not a career. With that in mind, senators should not be allowed to serve more than 12 cumulative years. I would define that as only being allowed to serve 12 years in the same position; so if you were in the House of Representatives and moved up to the Senate, you would be able to serve for 12 more years. After 12 cumulative years have passed, a life-time ban on serving in that particular position should be imposed.