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The challenger to Wisconsin Gov. Scott Walker will get a high-profile campaign visit from former President Bill Clinton on Friday, part of a last-ditch effort by Democrats to prevent the GOP incumbent from becoming the first governor in U.S. history to survive a recall vote.

Though President Barack Obama hasn't gotten publicly involved in the campaign, Clinton is the third prominent Democrat in recent days to appear in the state on behalf of Milwaukee Mayor Tom Barrett. Tuesday's election marks just the third time any governor in the country has faced a recall vote, and the incumbent was ousted in the other two.

Clinton's visit is a nod to the importance of turnout for Democrats in Milwaukee, a stronghold for the party where Barrett must do well in order to win. Clinton was to appear at a morning rally with Barrett to encourage voters to cast their ballots early. Friday is the deadline for in-person absentee voting.

The recall was spurred by Walker's proposal last year that effectively ended collective bargaining rights for most state workers. Obama has kept his distance from the union fight as his own re-election efforts near this fall. Wisconsin, with its 10 electoral votes, is a key part of his path to victory.

The recall election has been unlike anything seen before in Wisconsin, with at least $62 million spent by the candidates and outside groups so far, based on a tally released Thursday by the government watchdog group the Wisconsin Democracy Campaign.

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    Walker was the top spender at $29 million with Democrats including Barrett spending about $4 million. Outside groups have spent $21 million and issue ad groups that don't have to disclose their spending have put in at least $7.5 million.

    Maryland Gov. Martin O'Malley, who chairs the Democratic Governors Association, fired up Democratic volunteers Thursday in Madison. His visit came after U.S. Rep. Debbie Wasserman Schultz, head of the Democratic National Committee, made a similar stop Wednesday in Racine.

    "We don't want to wake up after this election and say, 'If only,'" O'Malley told volunteers. "Now is the time to turn on the afterburners."

    O'Malley said he expected the race to be so close, it could come down to a couple votes in every precinct. A strong voter turnout effort could swing the election by as much as five points, he said.

    Walker has been leading in polls released by the Marquette University Law School during the past two weeks. The most recent one released Wednesday showed Walker with a 7-point edge, just inside the 4.1 percentage point margin of error.

    O'Malley, before he arrived in Madison, said the DGA has spent about $3.2 million in the race so far, which is more than it spent in the 2006 re-election win by then-Gov. Jim Doyle and more than it spent for Barrett in 2010 when he lost to Walker by about 125,000 votes.

    Wisconsin Republican Party spokesman Ben Sparks said it was appropriate for O'Malley to bring what he called his "tax and spend stump speech" to the state.

    While collective bargaining sparked the recall, the abbreviated campaign has focused on many other issues with Barrett and Walker jousting over how well the state's economy is performing.

    Walker two weeks ago released new numbers that showed a more than 23,000 job gains in 2011, a dramatic shift from other data that showed nearly 34,000 jobs had been lost. Barrett has accused Walker of "cooking the books," but Walker's administration said the new job figures were approved by the U.S. Bureau of Labor Statistics.

    Walker on Thursday released another television ad attacking Barrett over a report that Milwaukee police misrepresented the city's crime statistics.

    The Milwaukee Journal Sentinel found at least 500 cases in which aggravated assaults had been misclassified as lesser assaults. If they had been classified properly, the violent-crime rate last year would have risen 1.1 percent from the previous year's figures, instead of falling 2.3 percent as the police department had reported.

    "If Tom Barrett is willing to cover up hundreds of violent crimes in Milwaukee, what else is he hiding?" the narrator says.

    Jim Palmer, executive director of the Wisconsin Professional Police Association, called the ad a "blatantly outrageous and baseless eleventh-hour attack" meant to "scare, confuse, and distract voters."

    Walker and Barrett were meeting Thursday night for their second and final debate before the election.

    Walker on Friday was to campaign with South Carolina Gov. Nikki Haley, who has called herself a "union buster" and considers her state's low union membership rate an economic development tool.