"A year ago, all the talk was Obama could never win with high unemployment," the AFL-CIO's political director, Mike Podhorzer, wrote in a recent memo. "The early conventional wisdom went further, writing off working class voters, asserting that the only path available to President Obama was upscale voters in states like North Carolina." Instead, the demographic that should have been Obama's greatest weakness may be the one that saves the election for him.

Indeed, if the manufacturing states of the Upper Midwest end up forming a surprising electoral-vote firewall for Obama, insulating his reelection bid from potential losses in Florida, Virginia, and Colorado, he will owe a debt of gratitude to the unions, who have been working doggedly, and largely under the radar, on his behalf. In the last four days of the presidential campaign, the AFL-CIO says its 128,000 volunteers will have knocked on 5.5 million doors, made 5.2 million phone calls and passed out 2 million leaflets in six targeted states. In Ohio alone, the union has 20 staging areas from which to make those calls and send crews out to knock on doors.

On Saturday morning, Tim Burga, the wiry president of the Ohio AFL-CIO, stood under an alphabet soup of signs, unions being second only to the military in their zeal for acronyms: AFSCME for Obama-Biden, NATCA, TWU, AFT, UFCW. Like a boxer entering the ring, he had taken off his mustard-colored work jacket, revealing a black IUPAT tee beneath.

"We've been active for a good long time now," Burga said. "We need to leave it all on the field now for the next four days. Walk till our feet are hurting. Talk till our throats are scratchy. Go to the work sites, pull every last vote out that we can pull out." The 40 or so union members in the low-ceilinged back room, a group long on pot bellies and mustaches, nodded approvingly. There was also a woman in a T-shirt reading "Kicking Ass for the Working Class."

"And when we do that," Burga continued, "we're going to say to America: 'Ohio won this election! Labor won this election, and we did it for the working people!' Are you with me?" "Yeah!" the group shouted.

The unions' effort is much bigger than four years ago, Trumka says, because of the campaign-finance changes stemming from the Citizens United decision of the Supreme Court, which made it legal for unions to engage in political communication with the general public, not just their own members. "Trust me, it's a horrible, horrible thing, and it's corrosive to democracy," Trumka says of Citizens United. "The only good thing it did is, it gave us the ability to do a super PAC so that we could talk to nonunion workers."

The organizing effort in Ohio got a jumpstart a year ago, when the state's Republican governor, John Kasich, attempted to pass changes to collective-bargaining rights. Labor revolted against the legislation, known as Senate Bill 5, and staged a referendum to repeal it that ended up passing with 62 percent of the vote. The success of that effort laid the groundwork for this year's political mobilization on behalf of Obama, Senator Sherrod Brown (who is also favored to win, despite having tens of millions spent against him by the Chamber of Commerce and other interest groups), and other Democrats.

"Trust me, Citizens United is a horrible, horrible thing, and it's corrosive to democracy," Trumka says. "The only good thing it did is, it gave us the ability to do a super PAC."

I spent a couple of hours Saturday afternoon walking a mixed-race, working-class neighborhood on the east side of Columbus with Diana Vernon, a sunny 51-year-old head cook for a suburban school district. Canvassing is thankless work -- hours on your feet, trudging from door to door to remind people to vote, almost none of whom are home -- yet Vernon does it almost every weekend, and last week spent a full day in the rain distributing literature for a union-backed ballot measure.