By DOUGLAS BELKIN And CAROLINE PORTER

Wisconsin voters headed to the polls Tuesday to decide whether to recall Republican Gov. Scott Walker in a bitter dispute over his efforts to curb unions that could affect the presidential race and influence policy toward organized labor nationwide.
More than $60 million has flooded into Wisconsin, and prominent politicians from both parties have traveled to the state to campaign in what many have called the nation's second-most important election this year. Recent polls showed Mr. Walker with a slim lead over Tom Barrett, the Democratic Mayor of Milwaukee, with both sides girding for a photo finish.
The race has fueled emotions on both sides of the electorate, and officials expect turnout to be high. At the polling station at the Kenosha Public Museum in the state's southeast corner, about 25 people waited in line at 7 a.m. for polls to open--several times more than in a typical election, said Peggy Gregorski, a museum official.
"We're nervous," said Mary Fetherston, a 71-year-old who voted for Mr. Barrett. "If Walker wins, he's going to go full speed ahead." She and her husband, Gerald, a 72-year-old retired speech therapist, said collective bargaining when he was working for the local school district helped to safeguard their retirement. The couple had a "Recall Walker" sign in their yard that was stolen last month and have stopped discussing the race with Mr. Fetherston's brother and sister-in-law, who back Mr. Walker. "It's a civil war here, families and neighbors," said Mr. Fetherston.
Nearby, Arthur Ahlgren, a 57-year-old Walker supporter, started shouting at the Fetherstons. A onetime union worker who now lives on Social Security, Mr. Ahlgren turned on organized labor after he was laid off from his auto-industry job in 1987. He thinks the recall effort against the governor should never have happened. "The smartest thing is to keep Walker because he has balanced the budget and he knows how to spend money," he said.
After exchanging words, Mr. Ahlgren and the Fetherstons apologized to each other, then went separate ways.
The 44-year-old Mr. Walker, who took office 17 months ago, is only the third governor to face a recall in U.S. history. The previous attempt unseated California Democratic Gov. Gray Davis in 2003.
The Wisconsin recall is rooted in a law signed by Mr. Walker two months after he took office that forced government workers to pay for more of their pension and health-care benefits while also cutting most of their collective-bargaining rights. Mr. Walker said the cuts were necessary to balance the state budget and to minimize public-sector layoffs. His aggressive action and unflappability in the face of the protests has made him a star in conservative circles.
Unions said Mr. Walker made them scapegoats for problems they didn't create. Tens of thousands of protesters demonstrated for weeks at the state Capitol building in Madison. Union organizers used that anger to gather 900,000 signatures and force the recall contest. Mr. Barrett has chided the governor on the campaign trail for trying to be "a rock star to the far right."
The results of Tuesday's election will reverberate in other state capitals as politicians struggle to address shrinking revenue and public-pension shortfalls that economically insecure residents don't want to pay for with higher taxes. The response of voters in Wisconsin, a centrist state, to Mr. Walker's move is seen as a test of how far Republican leaders seeking to close those fiscal gaps can push public-employee unions, which have remained strong in recent decades even as private-sector unions have weakened.
The recall race also has turned Wisconsin from a state Mr. Obama considered a sure bet for his re-election campaign to a potential battleground state. Mr. Obama won Wisconsin by 14 percentage points in 2008, but in an update on his re-election efforts sent to supporters on Monday, Obama campaign manager Jim Messina characterized the state as a tossup.
The recall race has drawn enormous interest and cash from outside Wisconsin. Of the $63.5 million spent on the race by the campaigns or their allies, $16 million has come from conservative groups such as the Republican Governors Association, Americans for Prosperity and the National Rifle Association, according to government-watchdog group The Wisconsin Democracy Campaign. Democratic groups, including unions, have contributed about $14 million.
Mr. Walker himself has raised more than $30.5 million—two-thirds of it from outside the state. Mr. Barrett has raised $4.2 million—a quarter of that from outside the state.
Positions in Wisconsin have been hardening for months. A poll by Marquette University taken the last week of May found that just one likely voter in 50 didn't know for whom he or she would vote.
Officials are predicting that approximately 2.8 million of the state's 5.7 million residents will vote, compared with 2.1 million ballots cast in the 2010 election when Mr. Walker defeated Mr. Barrett by six percentage points to become governor.
Efforts by both sides to get out the vote could determine the outcome. Labor groups and their allies knocked on more than 300,000 doors during the last few days and placed more than 400,000 phone calls, said Brian Weeks, the assistant political director for Afscme, the state- and municipal-workers union.
Republican groups said they also have developed a coordinated get-out-the-vote effort, including a last-minute mail program financed by the Republican Governors Association that included a push to increase early voting, which ended on Friday.
—Colleen McCain Nelson contributed to this article. Write to Douglas Belkin at [email protected]