A conservative incumbent surged to a commanding lead in Wisconsin's hotly contested Supreme Court election Thursday, after a predominantly GOP county's clerk announced she had incorrectly entered vote totals in the race seen as a referendum on Republican Gov. Scott Walker's divisive union rights law.

Waukesha County Clerk Kathy Nickolaus said more than 14,000 votes weren't reported to The Associated Press on Tuesday due to "human error." Nickolaus previously worked for a GOP caucus that was under the control of Justice David Prosser, who was speaker of the Assembly at the time and who now stands to benefit from the clerk's error.

"This is not a case of extra votes or extra ballots being found," Nickolaus said. "This is human error, which I apologize for."

Nickolaus said the most significant error occurred when she entered totals from the city of Brookfield, a suburb of Milwaukee, but they were not saved.

Before the announcement, it was assumed 68-year-old conservative Prosser's race against liberal assistant state attorney general JoAnne Kloppenburg was headed for a recount. But Waukesha County's corrected totals gave Prosser a 7,500-vote lead, which is likely to stand if none of Wisconsin's 71 other counties makes significant adjustments while reviewing their ballots.

Opponents of the law that takes away nearly all public employee collective bargaining rights had hoped a Kloppenburg victory would set the stage for the high court to strike it down.

Kloppenburg's campaign manager demanded a full explanation of how the error occurred. Melissa Mulliken said an open records requests for all relevant documents would be filed.

Rep. Peter Barca, Democratic Assembly minority leader, said the mistake raises significant suspicion that could warrant an investigation.

"It doesn't instill confidence in her competence or integrity," Barca said.

Nickolaus was given immunity from prosecution in a 2002 criminal investigation into illegal activity by members of the Republican Assembly caucus where she worked as a data analyst and computer specialist. Prosser, who as speaker of the Assembly in 1995 and 1996 controlled the same caucus, was not part of the investigation. Nickolaus resigned from her state job in 2002 just before launching her county clerk campaign.

The corruption probe took down five legislative leaders, all of whom reached plea deals.

Prosser issued a statement saying he was encouraged by various reports from counties as they were beginning the process Thursday of verifying the votes. He did not specifically mention the Waukesha County change.

"Our confidence is high, and we will continue to monitor with optimism, and believe that the positive results will hold. We've always maintained faith in the voters and trust the election officials involved in the canvasing will reaffirm the lead we've taken."

The race was so close, despite 1.5 million votes being cast, that the lead flipped back and forth repeatedly on election day and in the days after as preliminary totals were checked and updated.

Walker told The Associated Press before details of the new votes were announced that voters will demand transparency.

"The overriding principle has got to be that every vote that was legally cast in Wisconsin needs to be counted," Walker said.

The surprise discovery of votes that could give Prosser the win and quash any recount before it starts already had liberal groups crying foul. Nickolaus has also been criticized by the Waukesha County Board for her handling of past elections and lack of oversight in her operations.

"There is a history of secrecy and partisanship surrounding the Waukesha County Clerk and there remain unanswered questions," Scot Ross, director of the liberal group One Wisconsin Now, said in a statement.

An audit of Nickolaus' handling of the 2010 election found that she needed to take steps to improve security and backup procedures, like stop sharing passwords. The audit was requested after the county's director of administration said Nickolaus had been uncooperative with attempts to have county experts review her systems and confirm backups were in place.

The Government Accountability Board, which is in charge of overseeing Wisconsin's elections, will review Waukesha County's numbers to verify the totals, said agency director Kevin Kennedy.

"We will conduct our own review of issues because we want to make sure that we are tracking every entry she made into our system," Kennedy said. "We're concerned when data is missing from a total. We'd like to make sure there is much better communication between their office and our office."

Kennedy said it was unfortunate the clerk didn't double check the data before releasing it to the press.

"Mistakes are never simple, they usually compound themselves, but these are the kind of mistakes we see happen, we just don't see them of this magnitude," Kennedy said.

Nickolaus said she didn't notice an absence of votes because her figures showed a 42 percent voter turnout, which exceeded the 30 percent turnout the county typically sees in spring elections.

"That was an amazing amount of votes," she said. "So I had no reason to believe I was missing anything."

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Bauer reported from Madison. Associated Press writer Dinesh Ramde in Milwaukee also contributed to this report.