Case, farm equipment makers sue over South Dakota law.(South Dakota Dealer Protection Act)

Case Corp. and three of its competitors are trying to overturn a new South Dakota law that makes it harder for farm-equipment manufacturers to clump underperforming dealers.

Case, its rivals Deere & Co., Agco Corp. and New Holland NV, and a trade association all are suing the state's governor and attorney general over the so-called South Dakota Dealer Protection Act. The suit, which was filed on Aug, 16 in U.S. District Court in Sioux Falls, S.D., argues that the law violates federal and state constitutions because it's too vague and denies manufacturers due process.

The law favors dealers at the expense of manufacturers, said Emmett Barker, president of the …