By Brendan O'Brien

MILWAUKEE (Reuters) - Wisconsin's controversial Republican Governor Scott Walker will face a recall election on June 5 over a new law he championed that strips public sector unions of most power, becoming the first U.S. governor to face a no-confidence vote in nearly a decade.

The five-member Wisconsin Government Accountability Board, which manages elections, voted unanimously on Friday to formally certify more than 900,000 signatures calling for Walker's ouster, setting the recall election in motion.

Just hours after the recall vote was set, a federal judge in the Wisconsin capital of Madison struck down two key parts of Walker's signature law curbing union power that labor unions had challenged in court.

A Democratic primary will be held on May 8 to choose Walker's opponent in the recall vote. The Democrat most likely to face him is Milwaukee Mayor Tom Barrett in a rematch of the 2010 election that Walker narrowly won.

Barrett formally announced on Friday that he would run for governor, saying Walker had divided the state.

"As governor, I will fight to restore collective bargaining rights, because it's the right thing to do, and it's necessary to heal Wisconsin," Barrett said in a statement.

No elected U.S. state governor has faced a recall vote since California's Gray Davis was ousted in 2003 and succeeded by actor Arnold Schwarzenegger. The only other governor to be recalled in the last century was North Dakota's Lynn Frazier in 1921.

Walker set off a firestorm shortly after he was elected in 2010, when he pushed a law stripping public sector unions of most of their powers through the Republican-led legislature.

The law forced local and state workers such as teachers to pay part of the cost of health insurance and pensions, and took away union power to negotiate wage increases beyond the level of inflation.

What most infuriated unions were provisions that required a vote of membership every year to keep union representation at a workplace, and which made paying union dues voluntary, not mandatory. Both of those controversial parts of the law were struck down on Friday by Federal Judge William Conley.

"Scott Walker's (union law) has been divisive, unfair, radical and offensive to the values of Wisconsin," the state's Democratic party chair Mike Tate said.

The law sparked weeks of pro-union protests at the capital in Madison last year, and Senate Democrats fled the state in a futile attempt to stop the measure from becoming law.

Walker said the measure was needed to close a budget gap and put Wisconsin on a firmer financial footing.

HIGH STAKES VOTE

The stakes are high for both sides in the recall election. Organized labor sees Walker's agenda as trying to bust unions and is concerned that if the law is allowed to stand it could encourage other states to do the same.

Already, Indiana has approved a so-called "right to work" law letting union members opt out of paying dues, and Ohio tried but failed last year to force through a law curbing union power.

Wisconsin was also expected to be a closely contested state in the presidential election in November. President Barack Obama last year openly supported the unions in opposition to Walker, and leading national Republicans have sided with the governor. Continued...