Though labor's electoral role is getting scant media attention, union votes are likely to be pivotal in the outcome of the presidential race.
Despite declining membership since the mid-1950s, unions provide key strength for Democrats, accounting for one-quarter of all votes in some recent national elections, with up to three-quarters of that vote going to labor-endorsed candidates.
When the Democrats won the House in 2006 and when they captured the White House two years later, without labor's votes the races would have been too close to call.
Labor's strength in key swing states raises the ante, which brings us to the second reason you are likely to hear about labor on Nov. 7 and the days following: Ohio.
We've been told frequently that no Republican has made it to the White House without winning the Buckeye State. But that understates Ohio's significance. Since the Kennedy-Nixon contest in 1960, no candidate from either party has gained the presidency without winning Ohio. In fact, over the past 16 presidential elections — i.e., all post-World War II contests — only John F. Kennedy won without Ohio's electoral votes.
It's not only the sheer numbers in Ohio — 650,000 union members and more than 1 million union household members — but also that these often are swing voters in a swing state. Economically progressive but more conservative on social issues, their votes are perennially up for grabs.
Indeed, their unpredictability reflects the conundrum facing anyone trying to gauge what role union voters will play this time. I could give you a half-dozen reasons why they will turn out in large numbers and overwhelmingly support President Obama. But I could give you equally feasible reasons why their turnout will be smaller than usual and their margin favoring the incumbent will be thinner.
Here is why their votes will boost Mr. Obama: Union leaders are doing their typically impressive education and get-out-the-vote efforts. The union-bashing Republican ticket of Mitt Romney and Paul Ryan repels union members. Political attacks on unions (highlighted in Mr. Ryan's home state of Wisconsin) have galvanized workers. Mr. Obama saved the auto industry and millions of related jobs. The president is fighting for the middle class. The economy and employment are trending upward, with millions of jobs added during the past couple of years and unemployment finally below 8 percent.
Here is why many union members will stay at home while others choose Mr. Romney: Mr. Obama hasn't kept his promise to sign into law the Employee Free Choice Act. He stayed away from Wisconsin after promising to march wherever labor needed help. Trade deals haven't been rewritten. His administration is full of Wall Street types. Mr. Obama's jobs record has been subpar. Middle-class incomes have fared poorly under his stewardship.
Meanwhile, Ohio's employment picture has bettered the national average, but is that the president's doing or will the Republican governor get credit? Also, Ohio voters dealt Gov. John Kasich a rebuke last year by overturning his restriction on public-employee collective bargaining, but that very day those same folks disapproved of Mr. Obama's health care law.
Although it's unclear how labor's vote will play out, either way it will help determine the election's outcome. Consider: If union households vote heavily and strongly favor the incumbent — i.e., producing one-quarter of the turnout with three-quarters of those supporting Mr. Obama — Mr. Romney will need 59 percent of all nonunion votes, problematic given the president's strength among ethnic minorities and women. If, however, the union share of voters falls to one-fifth and just two-thirds back the president, Mr. Romney needs only 54 percent of votes from nonunion households, a more realistic figure.
Who occupies the White House as of Jan. 20 may well depend on where labor falls on that continuum, nationwide or even just in Ohio.
• Philip Dine, author of "State of the Unions: How Labor Can Strengthen the Middle Class, Improve Our Economy, and Regain Political Influence," is a Washington-based journalist and a frequent speaker on labor issues.