The problem with this, is that it's actually not in any state's benefit to do so. If you're a winner-takes-all state, then the stakes are high, and the candidates need to pay attention to you. If you're a proportional state, then campaigning there is only going to net you a handful of votes - there'll be a large core that stays for each party, with a relatively small number of votes attached to the swing voters. There's no real incentive for the candidates to pay any attention to you whatsoever.
In order for that to work, it'd have to be federally mandated and enforced, which would be problematic in itself.