WASHINGTON — They are the people who knock on doors in the dead of winter. They collect signatures and pass out literature. And when a candidate needs to assemble a crowd, they are there to fill up an auditorium with energy and enthusiasm.

The foot soldiers of many political campaigns are union workers, and for decades they have been the province of the Democratic Party and the envy of Republicans.

Now, with two of the nation's largest and most powerful unions — the Teamsters and the Service Employees International Union — bolting from the AFL-CIO yesterday and other unions considering such a move, political strategists, politicians and observers say that Democratic candidates may suffer the consequences.

"Our goal is not to divide the labor movement, but to rebuild it," said Andy Stern, president of the 1.8 million-member SEIU.

The leaders of the breakaway faction said they are leaving the AFL-CIO because of what they described as the labor federation's ineffectiveness in stopping the long-term decline in union membership and making unions more relevant to the challenges of the modern workplace.

"Our world has changed, our economy has changed, employers have changed. But the AFL-CIO is not willing to make fundamental change," Stern said.

Labor leaders widely expect two other unions — the United Food and Commercial Workers and UNITE HERE, a union of hotel, restaurant and garment workers — to join the breakup. Together, the four unions represent nearly one-third of the AFL-CIO's 13 million members.

AFL-CIO President John Sweeney described the SEIU-Teamsters withdrawal as "a grievous insult to all the unions" and a "tragedy for working people, because at a time when our corporate and conservative adversaries have created the most powerful anti-worker political machine in the history of our country, a divided movement hurts the hopes of working families for a better life."

There is general agreement that splintering of the AFL-CIO has large implications for the Democratic Party, which depends heavily on unions for money and organizing manpower. There is, however, no consensus on the specific impact.

"When you're a minority party and your base divides in two with factional fighting and feuding, that can't be a good thing," said Doug Schoen, a Democratic strategist.

"I think it's a disaster for Democrats," said Steve Elmendorf, a former senior adviser to Sen. John Kerry during the 2004 campaign. "The labor movement has been in decline and if you divide it further, it's only going to make it worse."

But Jim Jordan, Kerry's first presidential-campaign manager, called the impact "unknown and unknowable."

"Predictions of a political apocalypse for Democrats are premature," Jordan said. "We may see a newly energized or creative and more competitive environment within organized labor, and that's a good thing not just for labor but for Democrats."

Democratic strategist Chris Lehane said: "The approach represented by progressive reform organizations like the SEIU represents the future — they grow in size, they have fresh ideas, they understand message in the media age, they connect with the middle class. These groups are on the right side of history."

One immediate political consequence is that the AFL-CIO no longer will be able to coordinate get-out-the-vote drives that include members of the Teamsters and SEIU, and their families. Under the law, the AFL-CIO can mobilize only voters who are in member unions.

Organized labor has not witnessed such a breakdown in solidarity since union leaders, impatient with the conservative ways of the American Federation of Labor, broke away in the 1930s to form the Congress of Industrial Organizations to represent millions of unskilled workers. The two organizations later joined forces again.

Sen. Edward Kennedy, D-Mass., one of labor's strongest supporters, sought to downplay the negative consequences of the rift. "It's important, clearly, but I also believe underlying the division, what united all the unions is stronger than what divides them," he said, citing common interest in pension reform and in defeating partial privatization of Social Security.

The AFL-CIO is the coordinating group for more than 50 unions with 13 million members. Those members are often at the forefront pushing Congress to raise the minimum wage, protect workers' safety and negotiate international trade agreements that do not harm American workers.

The dissidents say unions need to spend more money on organizing, to merge so smaller unions can maximize their strength, and to focus their organizing drives on core industries rather than scooping up any available workers.

Currently, the nation's unions represent less than 8 percent of the private work force, a modern-day low.

Still, union workers are among the most reliable blocs of Democratic voters, according to exit polling and data collected by the AFL-CIO.

Those data show that the percentage of union workers who voted rose at a time when the labor movement was shrinking. For example, 19 percent of all votes cast in the 1992 presidential election came from union households, compared with 23 percent in 1996 and 26 percent in 2000. But by 2004, with a huge overall turnout, that percentage dropped slightly to 24 percent.

Compiled from the Chicago Tribune, The Washington Post and The Associated Press