Hundreds of union workers and other supporters of a bill that would make it easier for employees to establish bargaining units gathered in a rally across the street from the Senate.

The event marked the launch of an aggressive effort by organized labor to gain congressional approval—and a presidential signature—for the Employee Free Choice Act, a bill that will be introduced soon.

The measure would allow workers to form a union by signing cards and prevent companies from demanding a secret-ballot election supervised by the National Labor Relations Board.

After the rally, workers were to begin delivering petitions they said contained 1.5 million signatures of support for the bill.

Also on Wednesday, the U.S. Chamber of Commerce vowed to ramp up its campaign against the legislation, and the National Association of Manufacturers sent 50 executives and other corporate leaders to Congress to lobby against it.

“This is a bill that will be no less than a firestorm on Capitol Hill,” said Randel Johnson, vice president for labor, immigration and employee benefits at the chamber.

During the last Congress, the legislation was approved by the House but was blocked in the Senate. Democrats there are hopeful that their larger majority, which is now 59 pending the final results of a Minnesota race, will help allow them to overcome a filibuster with 60 votes.

In addition, President Barack Obama touted the bill, which he co-sponsored in the Senate, during his campaign last fall. But since the election, Obama has been reticent about the legislation.

He didn’t highlight it when announcing Rep. Hilda Solis as his nominee for labor secretary. Solis declined to state a position on the bill in her Senate hearing despite having voted for it in the House.

The first employment bill Obama signed was a pay discrimination measure. He followed that by signing three executive orders that bolster unions at federal contractors.

But so far the new Congress lacks its version of the Employee Free Choice Act. Rep. George Miller, D-California and chairman of the House Education and Labor Committee, said the measure would be introduced shortly.

“Soon. No more than weeks,” he told reporters while making his way from the rally back to his office.

In his exhortations to the crowd, Miller said that the bill would enable workers to join unions without being intimidated by their bosses.

That would give them leverage to raise their wages, increase their benefits and get their fair portion of the gains from their productivity, which is now going to “corporate elites,” such as executives and shareholders, according to Miller.

“You’re entitled to share in your productivity, your creativity, your hard work,” Miller said.

Detractors assert that the bill violates workers’ right to a secret-ballot union election, making them vulnerable to intimidation tactics from union leaders. Supporters say the bill gives workers a choice between a secret ballot and a card-check process.

Opponents also worry about a provision that would send a first contract to binding arbitration within 120 days if labor and management cannot reach an agreement. They say that would give the government unprecedented power over pay, benefits and work rules. Advocates say the timeline ensures that the first contract won’t be delayed to death.

The bill is the top priority for organized labor. The AFL-CIO was not shy in pointing out the day after the election that it spent $250 million and set up extensive grass-roots networks to help elect Obama and other Democrats.

The Chamber of Commerce refuses to back down. Since last spring, it has spent $10 million opposing the card-check bill and will “substantially increase” that amount in coming weeks, according to Steven Law, the chamber’s chief legal officer and general counsel.

As the war begins, neither side is contemplating compromise.

“It will pass,” Miller said.

The business community also is digging in. “There is not a middle ground on this bill,” Johnson said.

To achieve their goal, business advocates will have to keep all 41 GOP senators on board for a filibuster or persuade some Southern Democrats from right-to-work states to come over to their side.

Both supporters and opponents of the bill say that building support will be more difficult this year because members know that if Congress approves the legislation, a Democratic president is ready to sign it into law. Without a presidential veto looming, there will be no “free vote.”