PORTLAND – Dozens of union members turned out Thursday to confront a national business group’s campaign to promote opposition to legislation that would allow workers to unionize without the need for secret-ballot elections.

The Employee Free Choice Act emerged as an issue in Maine’s U.S. Senate race after opponents launched a lavishly funded television ad campaign that criticizes Rep. Tom Allen for his support of the measure. Allen, a Democrat, is challenging Republican Sen. Susan Collins, who opposes the bill.

The legislation, also known as the “card-check” bill, would permit workers to unionize when they get a majority of workers to sign cards favoring a union. Allen said he is proud to have voted for the bill and called ads that have been running for weeks criticizing his position as “attacks launched by big corporations and the U.S. Chamber of Commerce.”

Representatives of the U.S. Chamber, which is mobilizing opposition to the change, rolled into downtown Portland’s Monument Square to press the case that doing away with secret ballots in union elections is undemocratic.

That message has been amplified in recent weeks by TV ads in which an actor who portrayed a Mafia leader in HBO’s “The Sopranos” pressures a worker to sign a union recognition card.

The campaigns of Allen and Collins have condemned the ads, which are financed by outside political groups. The U.S. Chamber of Commerce was not involved in the decision to make or run the ads, said Glenn Spencer, executive director of the chamber’s Workforce Freedom Initiative.

“Part of what the Chamber’s been doing for the past year is to focus on the grass-roots side of this, rather than on the paid media side,” Spencer said. He said his group has been meeting with local business groups around the country to educate them about the legislation.

“We found that about 93 percent of American voters had never even heard of this bill. And this would probably be the most significant rewrite of national labor law since the 1940s,” he said.

After the chamber group set up tables in front of its bus, Maine AFL-CIO members gathered next to it, carrying placards and chanting support for the legislation and opposition to union-busting tactics.

Allen said the bill “does not eliminate a secret ballot but simply creates an alternative procedure for working people to organize for better benefits and pay. Under the current laws, the employers decide when and if an election can be held. They often delay those elections for years and in many cases the election is never held at all.”

Rebecca Pollard, representing the Maine Democratic Party, said the legislation provides the option of a secret-ballot vote if 30 percent of the workers so choose. Pollard said the change is needed because the Bush administration’s anti-labor stance has made it harder for workers to unionize in the face of employee intimidation.

Liz Reilly, a chamber staff member who has been on the bus tour, said the show of opposition Thursday was the largest that her group has encountered since it left Atlanta on Aug. 8 as part of a 32-state tour.