Turnout is high in an election fraught with labor politics.
The first-term Republican was back on the ballot just a year and a half after his election. Enraged Democrats and labor activists gathered more than 900,000 signatures to force the vote after they failed to stop Walker and his GOP allies in the state Legislature from stripping most public employees of their collective bargaining rights.
Walker faced a rematch with his opponent from the 2010 race, Democratic Milwaukee Mayor Tom Barrett, who lost by nearly 6 percentage points. If he succeeds, Walker would become the first U.S. governor to successfully fend off a recall.
IN PICTURES: Showdown in Wisconsin
The recall effort began bubbling last year, shortly after the former Milwaukee County executive successfully pushed through the union rights law that also required most state workers to pay more for health insurance and pension benefits.
"I think most people are just happy to have the election over," Walker said at a suburban Milwaukee elementary school where he cast his ballot. "I think most voters of the state want to have all the attack ads off. They want to have their TVs back. They want to have their lives back."
Barrett, meanwhile, applauded voters for turning out in force and for being prepared to wait a while to cast their ballots.
"Obviously the lines are very, very long, which we take as a very encouraging sign. People are engaged in this," Barrett said, adding that the energy around the state the past four days has been "building and building."
Turnout was strong across the state. There were lines at multiple polling places, but no major problems were reported. Anyone in line after polls close at 8 p.m. can still cast a ballot, said Reid Magney, spokesman for the state Government Accountability Board.
Turnout was on pace to meet predictions of 65 percent of eligible voters, Magney said. That type of turnout is more typically seen in a presidential race.
Jeff Naunheim, a warranty analyst from St. Francis who voted for Walker, called the recall a waste of time and money. But Barrett supporter Lisa Switzter of Sun Prairie said even if the recall doesn't go Barrett's way, "it proves a point."
"People in Wisconsin aren't just going to stand by and let a governor take over the state and cut social services," said Switzer, an occupational therapist and single mother on BadgerCare, the state's health insurance program for the working poor.
Preliminary exit polls conducted for The Associated Press suggested the public's views on collective bargaining are deeply entrenched. About three quarters either strongly approve or strongly disapprove of the changes for government workers. Overall voters were about evenly divided on the question, with about half approving and half disapproving of those changes.
And most voters made up their minds about the candidates before the final ballots were even set. About 9 in 10 who participated in early exit polling said they decided before May.
Both sides hoped for strong support from their bases: Madison and Milwaukee for Democrats, suburban Milwaukee counties and the Fox Valley around Green Bay for Republicans. Other more divided parts of the state, like along the western border and south of Milwaukee in the Racine area, could determine the race.
Walker stands in unique company: He is only the third governor in U.S. history to face a recall vote. The other two lost, most recently California Gov. Gray Davis in 2003.
Walker, the 44-year-old son of a minister, has remained unflappable throughout the campaign. Along the way, he's become a star among Republicans and the most successful fundraiser in Wisconsin politics, collecting at least $31 million from around the country since taking office. That obliterated his fundraising record of $11 million from 2010.
Walker and Republicans outspent Barrett and Democrats $47 million to $19 million, based on the most recent tally by the government watchdog group the Wisconsin Democracy Campaign.
The governor wasn't the only politician up for recall. His lieutenant governor, Rebecca Kleefisch, and three Republican state senators also faced votes, and a fourth state Senate seat will be determined after the Republican incumbent resigned rather than face the recall.
Tuesday's vote also will have implications for labor unions and the presidential election in November. Unions have a lot at stake because they pushed so hard to force a recall.
Implications for the presidential race were less clear, but President Obama did not campaign for Barrett. Instead, the president weighed in through social media, tweeting his support for Barrett. His campaign emailed supporters, urging them to back the Milwaukee mayor. But the White House cautioned against drawing any national conclusions from the recall's outcome.
"A race where one side is outspending the other by at least a ratio of 8-to-1 probably won't tell us much about a future race," White House press secretary Jay Carney said Tuesday.
Republicans are hopeful a Walker victory would pave the way for Mitt Romney to win Wisconsin, making him the first GOP candidate to carry the state since Ronald Reagan in 1984. If Walker loses, most agree Obama will have an edge. Either way, the state is likely to remain in play.
IN PICTURES: Showdown in Wisconsin