(FORTUNE Magazine) – Despite constant talk about the unseemly influence of money on politics, campaign contributions alone don't necessarily determine elections. For example, even though business interests have outspent organized labor by 11 to one in recent national elections, the AFL-CIO is likely to send more than its share of Democrats to Congress this year, because it is better at getting its members to the polls. In an era of low voter turnout, showing up can mean more than anteing up.

AFL-CIO President John Sweeney says his organization will spend $40 million teaching union workers how to persuade fellow members to vote. "The AFL-CIO and its affiliated unions will be more active in this election than in any previous one," vows Steve Rosenthal, the group's political director. That's bad news for business, considering how effective labor's ground game has been in the past. Union members represent only 17% of the voting population. But in 1998, 23% of the people who voted came from union households, and Democrats gained seats in the House. This year, 40% of the voters in New York's primary were part of union families.

The corporate response to labor's influence has been weak. Big companies usually divide their donations evenly between political parties to avoid choosing a loser and paying the price. In 1998 a small band of business associations led by the U.S. Chamber of Commerce spent $2.7 million on "issue" advertisements for Republican congressional candidates, but even that minor effort has been abandoned this year. Although companies legally can encourage top-level employees to vote for specific candidates, few do, mostly for fear of running afoul of arcane election regulations or of angering employees by meddling in such a sensitive realm. "Labor has more leeway under law, but corporations don't take advantage of the authority they do have," says Cleta Mitchell, a leading election-law attorney.

Using money from anonymous donors, former aides to House Republican Whip Tom DeLay are organizing ground operations in dozens of swing districts around the country. But the work of their Republican Majority Issues Committee may be too little, too late: Right now, the odds favor the Democrats' narrowly regaining control of the House. If it happens, a major reason will be that in politics, as in war, you have to occupy the ground--and labor has the most troops.