COLUMBIA, S.C. - Groups on both sides of a proposal to give parents tax credits to help pay private school tuition have spent lots of money to sway the Legislature.

Few of the groups, however, will say how much they've spent or who is providing them with the money. Some did acknowledged spending about $800,000 on their efforts.

Some lawmakers say that secrecy needs to go.

"This thing has the potential for costing billions of dollars out of the state treasury, and we as legislators should know who's trying to influence our state," said Rep. Ted Vick, D-Chesterfield.

Supporters of the proposal say the tax credits would force failing public schools to improve. Opponents say the measure would taken needed dollars away from public schools. Last week, the state Board of Economic Advisors estimated the program would cost $231 million after five years.

House Speaker David Wilkins said he would be willing to consider requiring more reporting from groups trying to influence the legislation. "The more openness you have in the process, the better," Wilkins said. "This may be the genesis for legislation on that."

Ed McMullen, president of the conservative think tank South Carolina Policy Council, said no organizations like his disclose membership lists. The Policy Council is pushing for the tax credits.

The organization did say it has raised about $650,000 and spent about $545,000 on ads this year.

An opposition group, Choose Children First, also would not release its financial information to The (Columbia) State newspaper.

"We made the decision early on that if the other side is going to (keep it secret) we would not disclose," said Lee Bussell of Choose Children First. "It would be unfair to our donors."

Political campaigns must disclose who gives them money and how that money is spent, but courts have protected the rights of groups supporting a position to keep their fundraising information secret, said John Crangle, executive director of the government watchdog South Carolina Common Cause.

"It would be nice if these organizations on their own would disclose their donors," Crangle said. "It's a nationwide problem."

Some out-of-state groups were visible in November's election, spending thousands of dollars - mostly unsuccessfully - to try to get pro-tax credit candidates elected in Richland, Charleston and other counties.

For example, Michigan-based All Children Matter spent $100,000 on advertising against state Sen. Joel Lourie, D-Richland, in the week before the election.

On the other side, the National Education Association has given a grant to the South Carolina association to advertise against the bill.

The money has paid for billboards and radio ads criticizing Gov. Mark Sanford, the bill's chief proponent, and the bill, said South Carolina Education Association president Sheila Gallagher.

The National Education Association "thought it was a bad bill," Gallagher said. "They had more questions than we have."

Groups do have to disclose how much they pay lobbyists. In 2004, four groups active in the tax credit debate spent almost $200,000 on their lobbyists. The most was spent by the South Carolina Association of School Administrators: $65,138 in the first half of 2004, according to The (Columbia) State newspaper.