The state A.F.L.-C.I.O. and two local unions filed a lawsuit Tuesday seeking to at least temporarily block a new law barring unions from requiring workers to pay the equivalent of dues. The lawsuit, arguing that the law is unconstitutional, was filed in Dane County Circuit Court a day after Gov. Scott Walker signed the measure. A spokeswoman for Mr. Walker, a Republican, and Attorney General Brad Schimel, also a Republican, both said they were confident that the law would be upheld, just as federal courts have ruled in favor of such laws in Michigan and Indiana. The unions say that the law is an unconstitutional taking of their property without just compensation and that enforcement would cause them irreparable harm.