Republicans have retained control of the Senate in the northern U.S. state of Wisconsin, winning four of six recall elections sparked by a contentious battle over public workers' collective bargaining rights.

The effort to remove the incumbents was linked to a budget measure passed earlier this year by both houses of the Republican-dominated state legislature and signed into law by Republican Governor Scott Walker. It strips most public workers of their rights to bargain on labor contracts.

Two Democratic senators face recall votes next week for the same issue, but after Tuesday's votes, they will still be in the minority even if they win. Another Democrat survived his recall election last month.

Before its passage, the collective-bargaining legislation generated angry protests at the state capitol and a walkout by Democratic lawmakers. Since its passage, the law has survived a legal challenge that went all the way to the state supreme court.

Recall votes are triggered when a certain number of constituents petition the state's Government Accountability Board. Officials must have been in office for at least a year.

Governor Walker faced outrage from Democrats concerning the budget measure. He took office earlier this year and thus was spared the possibility of his own recall election. The state's political climate could affect whether he faces a recall next year.