At least 10 states are now raising questions about the legality of the deal that Senator Ben Nelson, a Democrat, cut for his home state of Nebraska during the health care negotiations. Under the agreement, which is on the verge of being approved Thursday by the Senate, Nebraska is permanently exempt from paying for its expansion of Medicaid, shoving that cost onto taxpayers in every other state. Mr. Nelson was able to exercise such leverage because in exchange, he was providing the magical 60th vote that Democrats needed to advance their health care bill. The deal has enraged other Senators,...