In any event, such a prohibition would almost certainly be unconstitutional.
It could not, under current constitutional doctrine, be justified by a desire to prevent Wisconsin candidates from associating with out-state donors or to keep those donors from being heard on Wisconsin elections. It would be permitted only if courts could be persuaded that out-of-state money - by virtue of its origin and not its amount (out-of-state contributors are subject to the same limits as everyone else) - presents a larger risk of quid pro quo corruption.
The question answers itself. In federal elections, it's easy. A Member of Congress from Wisconsin gets the same vote on matters that affect Texas as one of its own representatives.
In state elections, it's just as easy. For someone to be able to get something from a Wisconsin elected official, he or she must have some interest in the state of Wisconsin. If that's so, then the risk of corruption is no greater than that presented by residents of the state - who also have interests in the state of Wisconsin. In other words, the threat of corruption is no greater with, say, the CEO of Georgia Pacific or a PAC associated with the national office of AFSME than it is with a local union or the CEO of Epic Systems.
If one is truly "outside" the state - i.e., someone with no tangible interest in state government - then there can be absolutely no risk of quid pro quo corruption. If George Soros or Sheldon Adelson have no business here, then they cannot benefit from state government. Their interest must be purely ideological, i.e., it must reflect a view about what is best for the state and, by extension, the country. If states are the laboratories of democracy and if a significant piece of national policy consists of the external effects of the policies of the individual states, it's not hard to see why a conservative or liberal donor in another state might care about what happens here.
Indeed, I suspect that much of the heavy spending from out-of-state donors on both sides is ideological.
When people criticize out-of-state contributions, they are really claiming that "outsiders" should have nothing to say about our elections. That strikes me as understandable, if a bit insular. But it's not enough, under our Constitution as interpreted by the Supreme Court to restrict the rights of expression and association involved with making and receiving campaign contributions.
Cross posted at Purple Wisconsin