Voters had no way of knowing the source of more than $180,000 that the Arizona Coalition for Education Excellence spent last year in support of school ballot measures. However, proving whether the coalition, or any other private corporation, violated the law is difficult.

Joseph A. Kanefield, who handles election and campaign-finance law at Ballard Spahr LLP in Phoenix and is a former state elections director, said that since federal court rulings two years ago, the increased influence private corporations have on campaigns and the decrease in disclosure is cause for concern.

"It gets complicated," Mr. Kanefield said. "They may feel that what they're doing is lawful but in reality they may be inadvertently violating campaign-finance law." The lure of non-disclosure often draws these groups to form, but "I'm not sure that's keeping in the spirit of campaign finance," he said.