What Sanders’s plan, as spelled out in his College for All Act, does is provide federal matching grants to help defray the costs of eliminating tuition for in-state students.
Specifically, he is offering a 2-to-1 federal match for states that do this along with meeting a few other criteria like reducing reliance on adjunct faculty. This is a sufficiently attractive offer that some states would probably go for it. But it’s going to cost a lot of money, and tax-averse Republican governors like Walker pretty clearly aren’t going to do it.