Burke, a member of the Madison school board, joined a unanimous vote taken behind closed doors Thursday night to have the board negotiate with the teachers' union on a contract extension.
Under the 2011 law championed by Walker, which spurred the protests and anger that forced Walker into a recall election in 2012, unions are barred from collectively bargaining over anything other than base wage increases no higher than inflation.
But Madison teachers are among a few statewide who have come up with a contract while legal fights challenging the law proceed. In 2012, a Dane County judge found the law to be unconstitutional as applied to the Madison teachers union and a Milwaukee public workers union. That decision has been appealed to the Wisconsin Supreme Court which has yet to issue a ruling.
The Wisconsin State Journal reported Friday that Burke voted with the board to begin talks with the union about extending the current contract through the end of the 2015 school year.
Joe Fadness, executive director of the Wisconsin Republican Party, said Friday that Burke's vote was a payoff to unions that have endorsed her.
In April, Burke won the endorsement of several large public sector unions, including Madison Teachers Inc., the Wisconsin State Employees Union and the statewide teachers union Wisconsin Education Association Council.
Burke won the endorsements even though she has failed to promise to repeal Walker's union law if elected. Instead, Burke has said only that she opposes the law, known as Act 10, and supports collective bargaining. She has also agreed with parts of Act 10 requiring public workers to pay more for their pension and health care benefits.
"Mary Burke made a desperate attempt to shore up the base that dislikes her with a cynical vote to violate the law and prevent taxpayers from saving money with Governor Walker's reforms," Fadness said in a statement.
Burke, in an email statement Friday, said she voted to enter negotiations because she supports collective bargaining.
"I feel that this is the best way to come to an understanding on the changes needed to move forward with the district's strategic plan and improve student learning," she said.
Attorney Rick Esenberg with the conservative Wisconsin Institute for Law and Liberty warned in a statement prior to the board's vote that extending the contract is "almost certainly illegal" and would likely be challenged in court.