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Union officials said the bill is needed to counter employer intimidation during organizing drives. Under the measure, workers could vote to form a union simply by signing a card, instead of by secret ballot, as is currently the case. It also calls for an arbitrator to impose a contract after 120 days if the union and management fail to agree.

Unions say the secret-ballot method gives companies the time to pressure workers and counter the organizing drive. The “card-check” method, they say, will make it substantially easier to force the company to negotiate.

Business groups counter that the secret ballot protects workers from intimidation from union supporters and fellow workers. Both sides agree that the change would greatly boost union membership, which stood at just 12.1 percent of the total U.S. labor force in 2007.

The chamber’s Mr. Donohue said it was clear the business community will be playing defense on a broad range of issues given Tuesday’s vote.

“I’ve been around a long time and I can count,” he said. “Given the makeup of the new government, it will be more difficult to advance certain business priorities and much harder to stop some anti-business measures.”

Both sides were trying to gauge the impact of the Senate vote, where Democrats have 57 seats with three races still undecided.

Jay Timmons, executive vice president of the National Association of Manufacturers, noted that some of the incoming Democrats, including Sen.-elect Mark Warner of Virginia, are not automatic votes in favor of the labor agenda.

Mr. Warner is “a perfect example of someone who hasn’t put a stake in the sand on this issue,” he said.

But Change to Win officials said several moderate Senate Republicans can be “brought around” on filibuster showdowns, and the movement plans to keep up the pressure in every state.

Asked whether the labor group was willing to postpone a clash over the organizing bill to spare the new administration a bitter political fight in its first 100 days, Mrs. Burger replied, “No. Is that clear enough?”