Sanders was an incredibly successful mayor before becoming a representative and then senator, and is a committee member on Committee on Veterans' Affairs, Committee on Health, Education, Labor, and Pensions, Committee on Energy and Natural Resources,Committee on Environment and Public Works, and a ranking member on the Commitee for the Budget.
Definitely a demonstration of executive potential, since he has superior access to competent advisers, proven track record of leadership, and a good working relationship with the legislator.
Really I think the only guy on the Republican side with comparable demonstrated executive ability is Rubio.