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Unions are facing a make-or-break moment in their campaign to drive Wisconsin's Republican governor from office.

If unions and their Democratic allies prevail in the recall — just over a year after Gov. Scott Walker signed legislation to curb collective bargaining rights for most public workers — it would send a powerful warning to other politicians who might try to limit union rights. Unions also might find it easier to turn out more voters in November for President Barack Obama in this battleground state.

A Walker victory would be a stunning setback for organized labor.

"If we lose, it's a shot in the mouth," said Greg Junemann, president of the Professional and Technical Engineers union and a Milwaukee resident. "We can survive it, but we'll be reeling."

Unions have experienced mixed results over the past year in trying to beat back efforts in dozens of states to restrict bargaining rights, pass right-to-work laws or limit how unions collect dues.

They enjoyed a major victory in November when Ohio voters in a statewide referendum repealed a law limiting collective bargaining rights for the state's public employees. But they fell short in an earlier recall campaign to wrest control of the Wisconsin Senate from Republicans and suffered a major defeat when Indiana this year became the first state in more than a decade to pass right-to-work legislation.

Recalling a sitting governor would be a major feat, something that's happened only twice before in U.S. history.

"After devoting so much effort, energy and funds to the recall, unions have to show positive results or it will be judged to be a sign of a weakened labor movement," said Gary Chaison, professor of industrial relations at Clark University in Worcester, Mass. "If they can't win in one of the most liberal states, where can they win?"

The recall primary in Wisconsin is May 8, and the general election is June 5.

Such massive, costly campaigns have taken a toll on unions, diverting resources they could have spent helping political allies or organizing new members. Unions spent nearly $30 million turning out votes to repeal the Ohio measure and more than $12 million on the Wisconsin state Senate recall effort.

"Unions are not bottomless wells when it comes to resources for this stuff," said former AFL-CIO political director Steve Rosenthal. "As we're forced to wage these fights to defend what we've got, it's a win-win for the other side because they force us to spend a lot of money to plug holes in the dike."

Unions and others had little trouble gathering more than 900,000 signatures to authorize a recall election for Wisconsin's governor. Finding a suitably pro-union candidate to take on Walker has not so easy. Most unions have lined up behind former Dane County Executive Kathleen Falk, who has pledged to veto any state budget that doesn't reinstate collective bargaining rights.

But unions now face a new hurdle in Milwaukee Mayor Tom Barrett, who announced last month that he, too, would run in the Democratic primary, even after union leaders tried to talk him out of it. Barrett, who narrowly lost to Walker in 2010, has stronger statewide name recognition than Falk, but he has clashed with unions in the past and refused to take the unions' veto pledge.

Barrett's entry into the race means unions will have to spend even more money to boost Falk's profile. It also puts them in a bind. They are reluctant to publicly attack Barrett for fear of damaging him in the event he wins the primary. Because their ultimate goal remains ousting Walker, they face the prospect of having to spend even more resources in the general election to support a Democrat they don't really trust.

Falk has won the endorsement of the statewide teachers union and the largest union representing public workers. No unions have backed Barrett so far, but the head of the Wisconsin Professional Police Association has praised Barrett and there could be a rift if some unions rally behind the Milwaukee mayor.

Wisconsin's largest public employees union got into trouble last week by directing its members to an Internet video that attacks Barrett and incorrectly implies that Barrett supported Walker's plan to curb bargaining rights. The Wisconsin chapter of the American Federation of State, County and Municipal Employees said it used poor judgment and acknowledged the video was "over the top."

Rich Abelson, executive director of Wisconsin's AFSCME District Council 48, suggested unions wanted to avoid negative attacks on Barrett. "Labor and AFSCME's main priority is to defeat Scott Walker," Abelson said.

Walker, meanwhile, is casting the recall as a battle with out-of-state "union bosses" who want to benefit from taxpayer money.

"This is about sending a message about don't mess with us or we'll take you out no matter who you are," Walker said in an interview with The Associated Press. "For all the talk about collective bargaining, for the national unions it's really about the money."

Walker has already raised more than $12.1 million for the race — the majority of it from out-of-state donors — breaking a previous fundraising record that Walker himself set in the 2010 governor's race. Unions plan to spend at least as much as they spent on the state Senate recall campaign.

Walker's proposal, which passed the Republican-controlled Legislature despite massive protests and all 14 Senate Democrats fleeing to Illinois for three weeks, targeted only public workers and exempted most fire and police officers.

It forced state and local government employees, including teachers, to pay more for health insurance and pension benefits, and stripped away collective bargaining rights except over salary increases no greater than inflation. It also did away with automatic dues withdrawals and forced annual votes for the unions to stay officially recognized.

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Hananel reported from Washington, D.C.

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