The legislation, passed 241-185 on a nearly party-line vote, would take away the right of employers to demand secret-ballot elections by workers before unions could be recognized.
"It's simply about establishing fairness in the workplace," said House Majority Leader Steny Hoyer, D-Md. Republican leader John Boehner of Ohio said the real issue was "taking care of union bosses."
The House action was the second triumph for labor following the Democratic takeover of Congress. The House and Senate also have voted on separate bills to raise the minimum wage.
Labor groups saw the Employee Free Choice Act as one way of halting the downward trend of union membership, now about 12 percent of the work force.
The celebrations may be short-lived. Senate Republican leader Mitch McConnell, R-Ky., has pledged to block the bill and the White House says President Bush will veto the measure if it reaches his desk.
The House vote was short of the two-thirds majority that would be needed to overturn a veto. Thirteen Republicans voted for the measure; two Democrats voted against it.
Labor groups contend that secret-ballot elections have become a means for employers to intimidate workers into rejecting unions.
"In the past few decades, labor law has been so twisted by corporations and their union-busting hired guns that it is now virtually impossible to form a union against an employer's wishes," AFL-CIO President John Sweeney said.
Business groups campaigned against the bill, saying it is an affront to democratic principles and would give high-pressure organizers unimpeded access to workers.
House Speaker Nancy Pelosi, D-Calif., said the bill was "the most important labor law reform legislation of this generation."
"It is about basic labor rights, about the rule of the majority free from intimidation and about protecting jobs," she said.
Boehner said the bill really was about "taking care of union bosses. ... This is an effort to help them get more members, to make it easier for them to sign them up and to intimidate them to sign cards."
The legislation, also called the card check bill, would certify a union as soon as a majority of workers at a plant signed cards authorizing it. Currently, employers can require elections, overseen by the National Labor Relations Board, on whether a union should be recognized.
The labor rights group American Rights at Work said that, in the run-up to such elections, 80 percent of employers hire union-busting consultants and 90 percent force employees to attend one-on-one anti-union meetings with their supervisors. The legislation would toughen penalties against employers who violate worker rights during organizing drives and set up a binding arbitration process to prevent companies from thwarting a new union by bargaining in bad faith on an initial contract. "In many places, when you exercise your right to organize, you get fired, you get intimidated, you get harassed, you get followed home," said Rep. George Miller, D-Calif., chairman of the House Education and Labor Committee. But Rep. Howard "Buck" McKeon of California, top Republican on the committee, said the bill would take away basic rights to a secret ballot. "That right is squarely in the cross hairs and this chamber is about to pull the trigger," he said before the vote. McKeon offered a substitute guaranteeing rights to a secret ballot, but it was defeated 256-173. Labor Secretary Elaine Chao, in a statement issued after the final vote, said it was a sad day when the House "votes to deprive workers of their basic right to a private ballot election." Labor organizations cite employer coercion as a major factor in the steadily falling percentage of workers who are organized. Union membership has dropped from 20 percent of wage and salary workers in 1983 to 12 percent in 2006. Discounting civil servants, that percentage is 7.4 percent, the Labor Department says. Unions lost 300,000 dues-paying members last year alone, said one opponent of the bill, Rep. Mark Kirk, R-Ill. "But just because only 12 percent of Americans now choose to pay into a union is no reason to attack our rights as Americans to a secret ballot." The White House made a similar argument Wednesday in issuing a veto threat. "It is a fundamental tenet of democracy that individuals are able to vote their conscience, privately, free from the threat of reprisal," the White House said. Sen. Edward Kennedy, D-Mass., sponsor of similar legislation in the Senate, said he would introduce it soon. McConnell, in a statement, indicated Senate Republicans would make every effort to stop its progress. "We will not allow the progress already made on behalf of U.S. workers to be undone," he said in a statement.